A-level grades in Wales this year will be no lower than those the pupils achieved at AS-level the previous year, the education minister has promised.
Kirsty Williams made the announcement after England and Scotland changed their grading systems.
If a student gets a grade on Thursday below their AS grade a revised one will be issued automatically, she said.
Pupils in England are being told their final results will be no lower than their mock exams.
The Scottish government announced on Tuesday that all pupils would get the grades predicted by teachers.
This year's exams were cancelled across the UK because of coronavirus, with governments having to find alternative ways to assess pupils' performance.
In Scotland 76,000 pupils had their results upgraded after being lowered by a moderating system which critics claimed was a "postcode lottery" as it linked students' results with their schools' past performances.
Earlier on Wednesday, the Department for Education in England announced a "triple lock" - so results will be the highest out of their estimated grades, their mocks and an optional written exam in the autumn.
In her statement, Ms Williams said the Welsh Government had to make sure such changes "do not disadvantage Welsh students".
"Students in Wales, and prospective employers and universities across the UK, can be assured that their A-level grades reflect their work and externally assessed exams," she said.
"Almost half the final grade comes from AS-level exams - this is not the situation elsewhere.
"Therefore, in building on that completed work, I am giving a guarantee that a learner's final A-level grade cannot be lower than their AS-grade.
"If a student receives a final grade tomorrow that is below that of their previous AS grade, then a revised grade will be issued automatically by WJEC."
Ms Williams said she had received assurances that "students can speak with confidence to their prospective universities regarding their A-level grades".
She said she had asked exams regulator Qualifications Wales and the examinations board the WJEC to "work closely with the qualifications bodies of the other UK nations as they develop their plans". `
The minister said she would ask Qualifications Wales to "move forward quickly on relevant adjustments to a Welsh appeals process as soon as these plans are clearer, in order to ensure Welsh students are not disadvantaged".
"All appeals will be free for Welsh students, to ensure there is no financial barrier to ensure learners feel their exam grades are fair," she added.
National Union of Students president in Wales Becky Ricketts said pupils who did not achieve grades they were satisfied with at AS-level would still be concerned about how their scores were calculated.
"It remains to be seen whether this measure will prevent a results postcode lottery as was seen in Scotland, and we will closely monitor the situation," she said.
Speaking later, on BBC Wales Today, Ms Williams said the Welsh Government would "continue to monitor" developments in England "to ensure that results are not only fair and robust tomorrow but also the same next week", when GCSE grades are announced.
Gareth Evans, director of education policy at the University of Wales Trinity Saint David, said her intervention on A-levels had been "inevitable".
"The promise of a free and extended appeals process is very welcome, and will go some way to alleviating the concerns of many pupils who might not receive the grades they had anticipated," he said.
"The fact that Scotland and England had diverged from agreed moderation practices made it extremely difficult for Wales to continue as planned."
What are opposition parties saying?
Welsh Conservative education spokeswoman Suzy Davies said it was "vital to see a safety net being implemented in Wales to make sure pupils achieved the grade they had worked towards".
"So tomorrow, I am pleased that pupils' final grades are guaranteed to be not be lower than their previous grade at AS-level," she said.
Plaid Cymru education spokeswoman Sian Gwenllian said the Welsh Government "will be marked down severely for leaving teachers and pupils in limbo - and their hard work, initially, unrewarded".
But she said it was "welcome that the minister has listened to Plaid Cymru calls for a free and independent appeals process".
Before the education minister's statement, Brexit Party Senedd leader Mark Reckless expressed concerns students had lacked sufficient opportunity to prove their academic progress because of Covid-19.
"We believe that the best option would be for the Welsh Government to offer A-level students the chance to sit an exam in the autumn so that we can give students a chance to get a more accurate grade and to maintain the integrity of the exam system," he said.
Teaching union reaction
David Evans, Wales secretary of National Education Union Cymru, said: "Whilst it is disappointing that amendments to some grades have come late in the day, we hope that the Welsh Government proposals outlined by the minister will ensure fairness for young people about to receive their grades.
"To many young people, exam grades are a ticket to their futures, and should not reflect where you live, but what you are capable of.
"It has been an unprecedented year for these young people, and grades must reflect their true ability for there to be confidence in the system."
Dilwyn Roberts-Young, general secretary of UCAC, added: "Our members will welcome any steps to ensure that their students receive fair grades and that they will not compare unfavourably with their peers elsewhere in the UK.
"However, the fact that these changes are needed is a matter of considerable concern. These 11th-hour changes are bound to cause stress and confusion."