Plaster/Drywall repair question

Here's one for the experts...
I've moved into a 40 year old house which has plaster walls on gypsum-board lath. We removed what was essentially a closet over the stairwell in order to open up the area. So, there are three or four sections of bare wall (and ceiling) about 4 inches wide (the width of the removed walls) which need to be repaired. In addition, the interior walls of the old closet are not flush with the rest of the walls...they're set in almost an inch.
Questions: Since these walls are plaster, do I have to repair it with plaster or can I use sheetrock in the gaps that were left when the wall was removed? Same applies for the wall inset...when I bump that out, can I just shim out some sheetrock or should I put up new gypsum wallboard on top of existing plaster and put new plaster on that?
Basically - would the transition from sheetrock to plaster be noticeable?
Thanks for your help...I've debated this back and forth for a few months...
Greg
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I'm not an expert but I see no reason why new drywall wouldn't blend in well, especially if you are going to paint it. Why not do a small test. Put up a small square of drywall and see how it is. ds

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