UCB nuclear engineering traditionally focused on nuclear reactors for power generation, but has added some courses on nuclear medicine applications. Was either of these an academic or career interest?
Change of major within engineering at UCB and CPSLO:
To change major to applied math:
UCB: change to L&S as described at Change Colleges Into Letters and Science | L&S Advising , then declare applied math as described at Major in Mathematics | Department of Mathematics
CPSLO: general change of major procedure is at https://advising.calpoly.edu/change-major ; contact department for specific criteria. Note the unusual lookback policy that checks whether the student would have been admitted to the major as frosh.