If that kind of change is what is going to work best for you three, and from what you have written it sound like it might, then why not? I would suggest that the three of you talk it through calmly and see what the new configuration will look like in terms of quality time and the like.
Are the three of you living together and would that continue, if so? Do you share financial obligations and how would that change?
So much of this depends on your specific circumstances.
As for you not being "truly poly", I looked up the definition in the online tomes and I don't think that you are violating any of the long list of requirements for being truly poly.... ok that was me being sarcastic - if you love more than one (whether it's about a specific "more than one" or a general capability) I think that by most people's definitions of the word you are "true poly" - but I wouldn't sweat it - there's no "poly accreditation" that you need.