Good Morning and Welcome to the Forums!
Depending on the flavors of juices, I don't have too much trouble simply putting a different juice in a previously used tank. HOWEVER - this is all at your taste's discretion. Since I vape a bit (o.k. A LOT) at work, I'll rinse my heads and my tanks between fills. The first couple hits after refill are warm and a tad bit watery, but not too bad. I do have a steady supply of tanks through which I rotate. If you think the flavor change is going to be too drastic, simply pop the used head (and rinse it well) and then pop in a new, clean(er) head.
I've been doing this for weeks, and only Tuesday did I get brave enough to dryburn my coils. I didn't need any tools (pair of tweezers would have been nice, but weren't 100% necessary) Even dryburned some of my "work" coils yesterday.
So, long-winded "non-answer", clean if you don't like the "mystery flavor" between juice changes. At least rinse and dry, if you'd like. Dry burn, if so motivated, or simply change heads (probably easiest, if you have the supplies at hand)