just add a zero ... 18F4520 ... new part with fun new features too, but pin compatible ... there doesn't appear to be a low voltage offering of that part however?
There is a an LF4520, also a LF4620 which is even better.
To do something REALLY cool, check out the dsPIC33F line. 33F are all 3.3v Vdd-only parts. They're MUCH more powerful, generally much lower power, and actually cheaper. But it would require new code. Look up the dsPIC33F64GP706 or dsPIC33F128GP706.