So I spend some time on secondment in the D&A (data and analytics) group at Strategy& so can answer this from a first-hand perspective.
Truthfully, you don't need to worry about not being able to move to switch after you get onto the normal graduate programme. There is a lot of flexibility internally in seconding or doing projects with the Digital practice and certainly not a case of being "closed off" afterwards.
What I would recommend to you, is if you are applying to the normal grad programme, you should continue on that, and then do some projects once you join within Digital. That way you retain optionality and can also see if you like the work!
The issue with going straight in is, it's quite difficult (not impossible) to transition from Digital to Strategy but slightly easier the other way around. So unless you are certain you want to do data, then not the best option. Its a commitment to join straight in as (a) different compensation, (b) different career track, (c) specific sectors
Hope that helps, and feel free to ping me if you want to know more.