Most players seem to be using Forges, then converting them back when they need more items.
No building that has storage capacity (and that includes houses) can be converted. Think about it this way: let’s say you had a max capacity of 300 recruits and it was full. You attempt to convert a house to Alchemy Lab. What happens? Do you lose recruits? Are you allowed to temporarily exceed the maximum (the way you can with hero summons)?
Another common piece of advice will be to NOT build the Alchemy Lab. If you don’t know why, you should definitely at least skim this thread. The feature is widely panned, including by a player who built it to level 10 and accumulated 10,000 alkashards.