How to change a POI from Restaurant to Cafe

or can the category be changed from Restaurant to Cafe

Yes, it does make sense to change it in a case when it was previously predominantly a restaurant and is now predominantly a cafe.

If it's predominantly a cafe, then go ahead and change the tag from amenity=restaurant to amenity=cafe.

As far as adding new tags, yes, you can add a lot to give additional details. You can indicate the food options using cuisine=* and diet:*=* tags, and you can indicate the drink options (coffee, alcohol, and others) using drink:*=* tags.

If the establishment operates in different modes at different times of day, you can describe that with the opening_hours tag, eg, opening_hours=Mo-Sa 07:00-14:00 "cafe"||Mo-Sa 14:00-23:00 "bar". There's also opening_hours:kitchen to indicate when food is served, if that differs from the general opening hours.

The tag bar=yes is often used to indicate that a feature includes a bar. It's most often used with restaurants and hotels, but it could be used with cafes too.

It would also possible to tag restaurant=yes, though it's not as common and not documented. Personally I don't think it's necessary, especially if the food options are indicated by other tags.

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Note that distinction between cafe & restaurant is somewhat fuzzy across the globe. The amenity=cafe wiki page shows a picture from Austria with white tablecloths and a uniformed waiter. This is a traditional European cafe, but I believe that many mappers, just looking at the picture, would call it a restaurant. (The waiter's only carrying drinks, though!)

Ultimately it comes down to how the most prominent use of the particular amenity compares with the general understanding of restaurant/cafe in that region.