How to replace bathroom exhaust fan

I am in the process of renovating my guest bathroom. The heat/fan/light combo unit looks like it is as old as the house (50 years!). I brought a replacement that I am going to put in, roughly the same size. The existing unit is mounted to the joists and I have no access from above. Is is best to just hack it out and try to keep the hole as close as possible to the current size. Or is it better to cut a larger hole around it, pull it out, and somehow patch it? Either way, what is the best way to patch the sheetrock when I am done?
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