I was wondering, if we buy the new book, will the old source-material still be available/usable to my players and I? 

I actually forgot the new book name but you mean the one with the reworked monsters and player races?

If i understood other posts from the forum it will be a new book and not an errata or something like that. Ao if you buy it you will have both but DNDBeyond does not know yet how they are going to implement it.

i would assume it will be in a ‘Tasha’s’ way

Quote from Eendmans >>

I actually forgot the new book name but you mean the one with the reworked monsters and player races?

If i understood other posts from the forum it will be a new book and not an errata or something like that. Ao if you buy it you will have both but DNDBeyond does not know yet how they are going to implement it.

i would assume it will be in a ‘Tasha’s’ way

 So it'd be something that is parallel/optionally replaceable traits and whatnot, that'd be cool. Here's hoping that'll be the way it actually goes. 

On the last two Dev Updates, Joe has said clearly that folks will not lose the stuff they’ve already purchased.  Exactly how the two versions of monsters/character races will live together in DDB’s database is yet to be revealed, but all the info we have at this point says that previously purchased races/monsters will not be overwritten.