What is the best way to programmatically convert Ensembl ids from one release to other? e.g. transcript ids from release 75 to 100
The ID history converter is available as a Perl script.
This script accesses these MySQL tables, so you could query these tables directly.
Note that the mapping session is only one release at a time, so you would need to run 25 queries to go from release 75 to 100.