How does completing a previous season and current season work?

Sorry if this has been answered. I only made it about 20 levels in S1, but am a completionist, and want to complete it. If I buy the pass to complete it, do I have to finish the current season first, or can I just choose to 'remove' it in some way, and alternate between the two.

And if i can alternate, is that an ongoing thing.

TBH, all I'm playing RN is Veilguard (for months), but am grinding to finish the seasons as needed, lol).