How malleable are Junior Biglaw Associates, and when do we stop being so malleable?

After articling, I was placed in my firm's M&A group when my first choice was my firm's Competition, Trade and Foreign Investment group.

I am just grateful to have a job to be honest since my student debt is quite serious - but I am worried that I am digging myself into a hole with each passing day of experience in M&A - although M&A obviously lends itself to a very generalist skillset - I am not gaining any expertise with CUSMA provisions or the Competition Act (for example) that is core to what I want to do. I can try to ask for work - but I am honestly slammed and don't know enough about deal flow to be able to predict when the down periods are for me to fit in some competition work (for example).

How easy is it to switch to a different practice group at a different firm at my level? When does such a transition stop becoming easy?