There will always be a section of any user base that will hate a developer if they dare charge for anything. (Even if (as per your proposal) the charge is voluntary and those who aren't prepared to pay can still hitch a ride on the coattails of those who do; which is the problem that shareware has always had.) The people who fall into this category expect to get paid for their own work, but in their view anything that is or can be on the Web should be free (as in beer, not speech). There's nothing you can do about those so I'd be inclined to ignore them; you're entitled to be paid for your time and expertise, since ultimately it's the only thing that any of us have to sell.
I get the impression that you intend to offer the enhanced version for free as well. I'm not sure how I'd feel about that if I was one of those putting my hand in to fund it given the "coattails" effect mentioned above. My feeling is that it would probably be more equitable for it to be "free" for those who helped fund it (yes, I know, it's not really "free" since in a way they paid for it in advance...), and for it to be available to others for a nominal fee. If you go down that path you will probably minimise the "hate" effect if you still offer a completely free, reduced-feature "express" edition, though.
However to my mind the key question is this... can you actually deliver? And just as importantly, can you support what you deliver? And in this I'm not talking about your technical skills (if you've written the thing in the first place we can take it as read that you have the skill set to do the extra functionality), but rather in terms of whether your other commitments in life will let you deliver.
As long as your app is free it's not unreasonable for you to address any bugs or issues in your own time as and when you can on the basis of "you get what you pay for". Once peoples' hands go into their pockets though, their expectations naturally escalate. Doubly so, I think, when they pay in advance for something to be developed. And if you aren't 100% certain that you can deliver these updates AND support them then it might be something better left alone.
This story is worth reading before anyone embarks on a crowdfunding venture. I'm not suggesting that it's typical but it does show how good intentions can turn into something that's toxic for all concerned.
I definitely wouldn't say that you shouldn't do it - if you can pull it off then users get a better product and you get a bit of income to compensate for your efforts - but you definitely need a plan before you go ahead.
The best way of determining how your user base will react, though, is to ask them. It's easy enough to put together an on-line survey and hook it up to a feedback page where users can post their thoughts on how they feel about the proposal. That's what I'd do first.