To mortgage a property a solicitor has to undertake to the bank to investigate the title, disclose any flaws to the bank/ or certify the title to the bank, cause searches to be made, draw down funds, and register the charge. This is a simplification of the process, anyone who has been involved in conveyancing will know that 'investigating title' is a huge amount of work.
To switch, exactly the same process except there is a further repayment of the old mortgage, vacate of the old charge step involved. So actually on a switch there is more work involved, not less.