If you come up with some other idea how the conversion could technically work ( like destroying the BTC by sending them to a random address in order to receive a new kind of coins ) I think that would be even more drastic because if people are willing to destroy their BTC in order to jump to some other currency, it means that BTC has already lost its value.
If the conversion happens by selling it means that BTC still has value and everything is fine in BTCland. Just that one user has switched priorities.
I think you have this backwards. If there is a newer, and much better, currency that everybody wants, then many people will want to sell their BTC to get the new currency. Nobody will want to buy the BTC. Why would anyone want to buy an inferior outdated currency that most people don't want to use anymore. With lots of people selling lots of BTC, and very few people (if any) wanting to buy it the value will quickly drop and everybody will be left holding worthless currency.
On the other hand, if the new currency is directly connected to BTC, and you can instantly convert any amount of BTC to a matching amount of the new currency simply by sending the BTC to a "BTC destruction address", then BTC will still have value (a value exactly equal to a matching amount of BTCv2). People will be willing to buy or receive BTC, because they know exactly how much BTCv2 they can get by sending to the destruction address without any fear of loss of value.
If on the other hand everybody agrees that BTCv1 must go and BTCv2 is the future, then BTCv1 must become worthless so that it does not threaten BTCv2. How to guarantee BTCv1 becoming worthless is however another highly nontrivial problem. I think.
If BTCv2 is determined to be objectively better than BTCv1, then there is no reason to be concerned about BTCv1 threatening BTCv2. Even so, by creating a fee free one way conversion that destroys the BTCv1 coins in the process of creating BTCv2 coins, the BTCv1 supply will dwindle over time while the BTCv2 supply grows.