ChiSau
If you're in contact with a ground station, never change frequency without telling the previous ground station (or asking permission if they're ATC and you're in their airspace).
In this case, you'd tell your radar provider you're changing frequency (e.g. "Request frequency change to Somewhere on 123.45 to transit their airspace, will return to you when clear"). I like to tell them that I'll return to them - often they'll tell me to retain my squawk if they know I'm coming back. Then contact the station at the airfield you're overflying. When you want to change back, tell them "G-ABCD clear of your airpsace, changing to Previous Radar on 112.34". Simple.
If you change frequency without telling your current station you're doing so, even if it's only temporary, you're asking for trouble when they try to contact you and you're not listening!
FFF
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