Question BA International Honours program

Got admitted to BA international Honours program at St Andrews with Econ major. The program is very exciting and I am looking forward to it. But some of the rules to the program read very restrictive in regards to the major or the program overall. Does anyone have experience how difficult it is to change the major? My second question to the program overall is what would if I complete my second year at W&M and would like to stay at St Andrews for year three and four? Thanks