Buying new house but keeping old one

Currently own house with outstanding mortgage but plenty of equity. Got a great tracker rate which I dont want to lose.

What I want to do is buy a new house but keep the existing to rent out. Is this possible?

How would I do this? I probably cant get too 'normal' mortgages....

Would I have to port my existing mortgage with me to new house (and add more possibly) and then convert my existing house to buy-to-let mortgage?