change or changes

Hi,

Should I use the word 'change' or 'changes'?

Joe changed his dress habit and got a raise this month. John changed his dress habit and got a bonus last month. Sam changed jhow he dresses and got a raise the month before last. It seems to me, a lot of changes have been occurring in the way how one dresses in our department and to that, I would say "a lot of change (or a lot of changes -- good, IMO) seems to be a positive thing for some of our friends in our department."