How easy it is to change phone plan provider?

I'll keep it short. I'm moving to Tokyo in 2 weeks and my relo providers are recommending me to use GTN mobile because i need a phone number to open a bank account and i need a bank account to register with traditional phone providers e.g. rakuten/ahamo(docomo)

i was recommended GTN mobile because they take foreign credit cards to start my life with there so that should help me settle with bank account needs after etc.

problem is GTN is a little expensive and I am looking at either Ahamo or Rakuten as my long-term phone plans

when changing providers in japan, how hard is it? can I keep my number from GTN when moving somewhere else? saves me the trouble from updating my phone number