I have two concerns
1) Currently I follow Ayatullah Khamenei however I want to change this to Agha Sistani as he has more English resources available for people like us who live in the west which may answer more of my questions and give me a better insight. What is the process of changing taqlid and am I able to do so ?
2) I don’t remember choosing a marja to follow when I turned baligh so do my wajibats count such as Salah and fasting as I didn’t follow any marja for quite a few years ? Later on I found out I had to choose a marja so I just followed who my mom followed so the choice I made was made quickly without any thought.

1- There is no official process. You simply start following the rulings of the new Marja’. You can do so if you believe the new Marja’ is more knowledgeable in Fiqhi matters. So yes it’s permissible to make the switch.

2- Your previous wajibat are valid as long as you did them properly according to the mainstream Marja’s