Free is free. If you have to replace the current rims/wheels, then why not use them till they go bad. Whether that is sooner or later than the experience of others? Are these just rims or complete wheels?

However, I don't think I'd be swapping if what is on the bike currently is round, rolls and is not worn out. A tad worn is what most wheels are after used a while. But they are a tad usable as well. I don't think any of us care if your wheels are mis matched. Of course if you are using it as a showpiece, then certainly everything should match.