How hard is it to shift career focus after your first Grad job?

I was wondering how much is at stake when getting your first grad job because it seems like your first job as a graduate basically determines what jobs you will be able to get in the future. For example, if I get employed in a general Mech Eng role with an oil company after I graduate, how would I get a job focusing on a specific part of Mech Eng like FEA, Thermo or Control if there was very little on these topics in my previous "general Mech E" role?