I chose a Marja based on his reputation for knowledge, but as a Baligh Shia, I am now reconsidering my choice. Are my past prayers and fasting valid? Am I allowed to switch to Sayyid Sistani or Ayatollah Makarem Shirazi?
Your previous prayers and fasts and worship are valid as far as your intention was for the sake of Allah, and you did not violate any basic rule of worship.
Taqleed is acting upon the verdicts if the most learned scholar (Al-A’lam) who is considered by the high caliber scholars in the Hawza who are known as Ahlul Khibra أهل الخبرة.
Switching should be to the most learned Al-A’lam.
Wassalam.