Are E & I and O & U Interchangeable in Pangasinan?

Are E & I and O & U Interchangeable in Pangasinan? (Orthography matters) A younger colleague once asked me, "Aren't 'i' and 'e' and 'o' and 'u' interchangeable in the Pangasinan language?" I instinctively answered "yes" with a knowing laugh. But upon thinking further on the matter, I noticed that native speakers actually distinguish between these vowels, although with the reckless substitution occurring more in writing. We say bigis instead of biges, begis, or beges. We say lupot, but never lopot, luput, or loput. We have a barangay called Dusoc, which we never call Dusuc, Dosuc, or Dosoc, although sometimes other people spell it as Dosoc, which for me is wrong.

We always say buro, never boro, buru, o boru. Same with puto. We say ubong and utong, not obong or obung or otong or otung. We say bubon, not bobon or bobun or bubun. We say uong, not oong or oung. We say tuo, not too or tou.

Admittedly, we don't use /e/ as much as /i/ because we substitute it with the schwa sound: /ë/ or /ə/. Nevertheless, in certain instances, we allow variations for bai and laki like bae and lake when speaking. Your thoughts?