I want to keep my current tmobile number while switching to a new family plan within Tmobile

I’m currently on my parent’s t-mobile family plan. I’m getting married soon and want to switch onto my future spouse’s t-mobile family plan. The rep stated “since it’s already a Tmobile number they can’t switch plans”. There must be a workaround for this… I’ve had this number for 15 years and I don’t understand why it can’t be a smooth transfer if I was able to keep this number going from ATT to Verizon to T mobile… Any suggestions for the phrasing I need to use/correct terminology?