As the title has changed to contain a specific question since I originally wrote this answer, I want to address the title question explicitly as well: no, I do not think it is appropriate to ever change authorship order without agreement among the authors. However, I think grant proposals are not the same as published work, and sometimes grants are governed by rules about who is allowed to be responsible. It's possible this was done as an administrative change, though the correct behavior would have been to discuss this change with the other authors before making it.
Is there any meaning at all to the order of authors in a grant proposal? I really doubt it. Therefore, I can't see any value/benefit your supervisor would get from making this change, and therefore no malice.
I'm of course familiar with designating some individual as a "PI", which often requires them to be a professor or otherwise 'permanent' employee of an institution (or, alternatively, requires them to be a degree-seeking student or postdoctoral trainee), of course. The grants I contribute to have only the PI listed as though they are an "author"; everyone else is listed as some other type of contributor. The PI is responsible for administration of the project (boring admin stuff: budgets and assurances and regulatory compliance); they may not do all or most of the actual work, and very often have a smaller percentage of their salary covered for a project than students and post docs do who are working directly on a project.
My best guess is that, even though no instructions were given about author order, your supervisor assumed that the PI needed to be first, and made that change. I don't see, from the information I have here, a reason to consider this aspect to be any sort of willful violation.
If you were on better terms, I'd recommend simply asking for an explanation (in a non-accusative way), e.g., "I noticed you've changed the order of authors; is it necessary for the PI to be first author? or was there some other reason for the change?" That conversation might also be a good time to raise issues of authorship order for papers that come out of the project, where (field-dependent) that order does matter.
Since it seems like you have other conflicts with this person, well, I can't tell you how to judge them overall, but I would recommend basing that evaluation on everything else you know instead.