Jeff- If there is an old crack under the new plaster, the new plaster will also crack. I would put up new blue board if you are going to plaster. I believe the 1/2 inch board 12 feet long is easier to handle than the thinner and shorter board. You will need to extend the electrical sockets but this is simple. You will need new trim. You can hang the boards your self or hire a 3 man crew that can do a large room in two hours including a 15 minute beer break. They work cheap and will make a great mess.
If you don't want to go to this trouble, just use joint compound to cover the cracks and paint the wall. This will look good for several months to a couple of years.
I have heard of some type of fiberglass mesh that can be glued over the existing whole wall. I have water damage around all six chimneys in this 12,000 sq foot house amounting to between 28 and 32 feet per wall by 9 feet high. With the roof completely done no more water intrusion hopefully in my lifetime. I have been using the Pro-kote system for crasks 30 years with great results. The owner does not want drywall as the whole house is plaster. Thanks , Jeff
Jeff, If the original damage is from water, and not expansion and contraction, you don't have to be as picky about the cover up. I'd apply 3/8" denseglass and replaster the entire wall or ceiling. You could use cal-board too, then apply a thin coat plaster. There are all kinds of small plaster contractors looking for work.
I use my finger nail to open my beer, lift and fold the loop back. I also have been in the drywall/plaster trade for 40 years. If the plaster on the wall is nice and solid you may be able to tape and coat any cracks with drywall joint treatment. If the plaster is textured, that some times can be hard to match. Texture with several coats of paint always seems to look differnt than texture with two coats of new paint. If the existing plaster surface is loose excessily cracked it is better to resurface with new drywall or blueboard/plaster for a permanent repair. A lot depends on how fusy you are.