Unit problem

I am contructing a map where a unit has several choices on what to upgrade to. The way I was attempting to do this is to sell a unit from the unit that is upgrading and then remove the original unit. The problem I am having with this is that when selling a unit it does not recognise that the unit has a requirement that must be met before it can upgrade to the other unit. Any suggestions on how I can make this work? I am willing to do trigger upgrades (already have a few on the map) but I cannot make the unit sell them?

Sorry if this is confusing. if you need clarification ask away and I will provide more details.

NVM, got the answer elsewhere.