This is most commonly associated with the verb “to do”

” I would change my personality if only I could.”( do it )

This phrase states that “My choice will be to change my personality in the future if I am able to do that thing”

The same spelling would be used for these two words in a very similar phrase using the verb “To have” but refere to an event that has already passed.

“I would have changed my personality, if only I could have” ( done it)

I think the gramatical terms for the difference in the meaning is most easily understood by thinking of the first explanation being a projection of what might happen in the future if you took this action” I would change my personality…”. The second example shows that the action was not taken and it occurred in the past.” I would have changed my personality..”

“I will go to Paris, and while I am there I could visit the Louvre. That would be interesting.” Sounds OK to me.

we could have been way more rich by 2008