Is using temp parts a bad idea?

I’m planning to upgrade from a kind of junky prebuilt into a custom built system. However GPUs will be a bit hard to come across for a bit due to obvious reasons.

I was thinking of buying everything and just stealing the 3070 from my prebuilt to use as place holder until a 5080 is reasonable to get.

Is it a good idea to scavenge my prebuilt for place holder parts so I have a useable system? I might also try to move my ssd over as well to save a bit of money and avoid trying to redownload everything.