...... no.
There seems to be some confusion going around with wirefly/lets talk. I even saw a post where someone thought they were signing a contract with wirefly/or lets talk AND Sprint. It's not exactly like that.
The only thing to avoid doing is to buy the phone on a new line (or upgrade) then move the phone to a different line. Wirefly/LetsTalk says they consider this a termination of service (if done within 6 months of purchase) and will hit you with ETF. They do this (again, they say, never witnessed it personally) because the phone needs to remain activated for 6 months for them to get "perks/credit" for the activation.
Otherwise, you can do whatever you want with your Sprint account. Add a discount if eligible, change your plan, add/remove service addons, etc etc.