It would be fairly normal, I am assuming you wish to take over the whole mortgage in your name only, if that is the case then you are effectively applying for a new mortgage, submitting income details etc and it will go through their underwriting procedure and will require a new loan offer etc. Have they already indicated you would qualify for this?
Removing a name from a mortgage for example if one party died would not require a new loan and existing one could be amended.
If in reverse you were adding a name some banks will just straightforward add the name to the existing mortgage which does not require a new loan offer and does not lose the rate.
However in the present climate I am sure all of them will do anything they can to get rid of a tracker mortgage.