If Trump goes to prison, would the Secret Service follow him? Sounds like yes.

Under the Former Presidents Act, which was signed into law in 1958, all living former presidents are entitled to receive lifetime Secret Service protection unless they decline protection.

It would require a change to Federal law via Congress to modify this entitlement. In the 1990s Congress did change it to a 10 year limit post-presidency, but they removed the limit again ~15 years later.