Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. immūtātus (inm-), a, um, adj. [2. in-mutatus], unchanged, unaltered (rare but class.): id mutavit, quia me immutatum videt, Ter. And. 1, 5, 7: veritas est, per quam immutata ea, quae sunt aut ante fuerunt aut futura sunt, dicuntur, Cic. Inv. 2, 53, 162.
2. immūtātus, a, um, Part., from immuto.