Putting new SIM into older phone to see if old Sprint service plan is as promised?

I’m one of the Sprint customers who t-mobile/Sprint has been warning us for months our particular older phones will stop working soon.

I have my wife’s  SONIM XP3 working  on a current valid t-mobile account,

T-mobile just sent me a new Actel GoFlip (to replace MY old Sprint flipphone which they say will stop working on the network soon.)

The new phone comes with it's own SIM and I could activate it the ordinary way, but as soon as I do my old Sprint phone , which I love, would stop working. immediately, and likely could never be reactivated. (I DO understand I COULD just do an ordinary activation of the new phone on my own account and have 14 days to return it if I don't like it, but as I said that would burn my bridge back to my current working old Sprint phone… which as far as I can tell will likely keep working for at least two more months despite t-mobile's dire warnings.)

I have a very unique older plan (FWIW it's called "free and clear") on the old Sprint phone. T-mobile has promised me that when I activate the Actel that will be preserved with all its features. I don't completely trust that because I've had inconsistant answers from t-mobile on that.

So BEFORE activating the new phone I'm thinking of doing this: Put the new SIM (which should be associated with my current Sprint.t-mobile phone number? right? Or not?) into my wife's working/activated SONIM and see if the connection/service has my accounts expected/promised service plan features.

Think that would work? Or is that new SIM actually a blank slate not associated with any account yet?

Any harm in TRYING that swap?

Alex