Yes. With most providers there are two options to do this.
The first is simply to just make the new contribution via Cavendish Online. In this case only that new contribution will be on the reduced Annual Management Charge (AMC) and your existing pot will be unaffected (presumably remaining on the normal, higher AMC).
The second is to set up a new plan via Cavendish Online and to simultaneously transfer your existing plan into the new one. This way your whole pension will be under Cavendish Online’s reduced charges. This process is called 'repensioning'.