Looked and looked, can't find a setting anywhere for this. How do I make it faster/slower to switch between siege mode and tank mode?
None.
I do stuff and thingies... Try widening and reducing the number of small nooks and crannies to correct the problem.
Data -> Abilities -> siege & unsiege abilties -> "Info +":
Change the durations.
You should change the animation speed of the "Siege Tank Sieged Morph" model, if you want to change the animation speed, too.