Sure, no problem!
By default we'll leave the original sipgate number on your account after your porting number has transferred over to sipgate basic.
Like any sipgate basic telephone number there's no recurring service charges for your original sipgate or ported telephone number.
You'll be able to select either number as your outbound caller ID.
Currently sipgate basic accounts are designed to support one phone number only, so inbound calls to either number will ring through to all connected VoIP phones in the same way.