Hmm, so I've looked at the wiki and the discussion you mentioned, and I see that this is an annoyingly complicated issue. It seems there are two main camps - the camp that says old_name is better and was:name has no place, vs. the camp that says these two tags mean different things. The case for the former camp seems to be that (A) old_name is more widespread already anyway, and (B) Nominatim doesn't pick up was:name diminishing its potential usefulness.

To the latter camp though, old_name is for when the current object changed names, but is still the same thing, while was:name is for when it changed into something different, not just changed names. If we were to adopt that style, then this building definitely should use was:name, not old_name, because it's not a Christian church anymore, it's a Guyanese Hindu temple.

What do you think about this though? It seems like you're more in the camp that says just always use old_name for stuff like this.