Warlock is time limited. Titan is suppose to be in the store permanently.
Modules comes later but CCU's should be available so you should be able to effectively "upgrade to a warlock" from a Base Avenger but you can't downgrade to a Titan (at least not yet)
Modules comes later but CCU's should be available so you should be able to effectively "upgrade to a warlock" from a Base Avenger but you can't downgrade to a Titan (at least not yet)