What happens to the FORTNITE skin change (tasks) after the season has ended?

Yes on the BattlePass you get these skins assigned to tasks (with so many XP you can change the skin like that)

I have not played since Season 5. The lvl 100 skin and lvl 1 skin from BP4 and their tasks (with the ability to level these skins) was taken from BP5. - However, I have now, after several months still not play the tasks from BP5 because (now it's BP8) - now to my question: When the Season 8 is now ready. Do I still have the chance to level my Season 8 skins in season 9? Or was that with the Drift Skin only once

If I understand your question correctly, yes. In each season you can level up the BP skins of past and current season.