How to transition to vegan?

In September last year I switched to veganism cold turkey bc I wanted to lower my cholesterol. After four months I had to stop bc my intestines could not handle it...unending bathroom trouble.

Yeah, cold turkey switch was not good. I want to try again.

1. Can a 60 y/o person modify their microbiome to adapt to vegan diet? Or is the pattern of lifetime omnivore too hard to overcome?
2. Could someone suggest a resource that could guide me through a sensible transition?

TIA.