what happens to the value of the coins that you purchased using bitcoin?
Are you asking about the price of altcoins that you bought with bitcoin after there's a hypothetical crash of BTC?
If BTC drops like crazy and the altcoins don't, then you saved yourself from the drop I guess. I don't see what other conclusion to come from in that scenario.
In the case that the altcoins you put your money into as well drop, then you'll probably see a worse drop than BTC if the entire crypto market is just plummetting.
Honestly, these are very tentative and probably not likely to happen soon.
It is necessary to wait if conditions permit.