reformist

Short definition from Wiktionary

Adjective

1. 1
   Advocating reform of an institution or body.
2. 2
   Specifically, advocating reform and the gradual accumulation of small changes, as opposed to revolutionary action.

Noun

1. 1
   One who advocates reform (of an institution).
2. 2
   Specifically, one who advocates reform of society and the gradual accumulation of small changes, as opposed to revolutionary action.
3. 3
   An advocate of reform in the Church of England; a Reformer.
4. 4
   An advocate or supporter of political reform in the United Kingdom. (Common from ca 1790 to 1830.)
5. 5
   A member of a reformed religious denomination.