The advice often received and piped to such change is: Shock -> Resistance -> Acceptance -> Commitment
Expect to be told something similar to this. Check yourself, is this what's happening?
On finding a manager:
Report to and build an effective relationship with the most senior person your can find. For manager tier in a large company this is probably an MD, D or Partner. Gain their trust. Get a couple more as mentors. Make sure there are 2-4 levels of corporate hierarchy from you and them. This removes any kind of hostility they may have from yourself appearing as competition. For a small company I guess this means report directly into a Founder. If you're 50-60 people, and already managing a team, are you in this position already? If not your org is already way too hierarchical.
Being a remote worker, add value:
A remote worker is often seen as a function. Give them something to do and they do it. Same is true of branch offices. Get closer to headoffice and promotions happen faster as implicit exposure happens faster. This is a downside of remote working. In what way are you contributing that an outsourced provider cannot?
Being a manager is not being an expert:
Promoting someone with niche knowledge to management doesn't seem to make a lot of sense. It is a manager's job to manage, not be an overbearing expert, that's what delegation and judgement is for. Is there anything else going on?
What I'd say:
If you really are happy in the job and would like to stay, I'd ooze maturity. Don't say this, as I don't know it fits your exact situation, but if it is like I'm imaging it, I'd change it to an opportunity to build a closer relationship: "I know I managed XXX before, and now they're managing me. I just want to say that's fine as I find value in doing YYY, the core part of my job, and you don't need to worry about any hostile reaction from me."... <<reaction>> [Get some opportunity to talk about my own plans for the coming year, basically a hook I can use to check-in with them in the future.] Don't do this in email. Skype face-to-face at the least.
Relationships are key:
And, for anyone working remotely, spend huge efforts to not just send a tracker of what you've done, but have a regular check-in and 1-1 every week with your boss and anyone else you find valuable.