A tax return can be amended up to two years after the due date of the tax return, for example, a 20/21 tax return can be amended up to 31/01/2023. If your first year of trading is 20/21, then you can amend the returns to change the accounting period.
With reference to early years loss relief, the same time limit applies, therefore this can only be claimed for years in which you can still amend your tax returns. You can see more information here:
BIM85045 - Trade losses - types of relief: relief for losses made in early years of trade
HS227 Losses (2022)