Yes, the first one that you've said is ture. It is understood when someone says "Ben değiştim" as someone changed himself/itself/herself, not about a new set of clothes. If it was understood with this meaning, then the sentence should have been "Ben geçen yıl kıyafetlerimi değiştirdim". I hope it helps to understand. :)
'Geçen' means 'last' (as in it has past).
Eg. Geçen yaz Fransa'ya gittim = Last summer, I went to France.
However ... 'Geçenlerde' means 'recently'. Eg. Geçenlerde sinemaya gittik = We went to the movies recently.
'Son' also means last (as in final, the very last) Eg. Yazın son günleri = The last days of summer.
The use of 'son' and 'geçen' can both mean 'last' but which one to use will depend on the context.