Of bigger concern to me is health issues due to insulation entering living space. Optimum solution is replacing old insulation with foam. Aside to my concern with breathing insulation,blowing additional loose insulation over what's there would be cheapest and stop most air flow. UNLESS you or someone pulls insulation back in sections,lay vapor barrier down,push insulation from adjoining area onto barrier then lay barrier ecet ect.

Thanks for your response. I am also concerned if insulation is entering from the attic.
What type of foam are you referring to for replacing the attic insulation?
If I lay vapor barrier, do I staple the sides of it to the ceiling joist? Also, is there any concern for moisture being trapped between the shiplap and the vapor barrier?

Opinions differ on how to position vapor barrier and whether barrier is even needed so I suggest you research the subject and make your own decision. Food for thought,most homes today are wrapped in Tyvek or similar and older homes have barrier on walls facing living space. Loose blowen without barrier in ceiling is popular in both older and modern homes. You will get all three opinions in forums like this so I reccomend you also consider what is and has been done in the field. As for stapling,if you install barrier it will be far quicker using rolls covering joists along with space between so no staple is needed. Trapped moisture questions should be answered by above research,I have no desire to debate the subject with houzzers and their interpretation of code. I will tell you my home is from 1983,in rather humid North Texas with loose blown on drywall ceilings and never a problem with moisture,mold or mildew.

The foam I'm referring to is sprayed onto surface ,drys to look and feel exactly like a Styrofoam drinking cup. This has become very popular in new construction sprayed on underside of roof decking and nothing is used on ceiling where your insulation is now.

Styrofoam insulation is polystyrene. It's a rigid foam insulation which can be of extruded or expanded polystyrene. Most spray foam insulations are either low or high density urethane foam. They're not the same.

Once cured, spray foam insulation is rigid; it won't move. Your wood ceiling will move seasonally in response to changes in temperature and relative humidity. You'll need to de-couple the insulation from the wood ceiling. You might consider rigid foam insulation between the rafters with spray foam insulation applied to seal the rigid foam to the rafters. That would function both as an air seal and vapor retarder and still permit the wood ceiling to expand and contract. You could install additional insulation above the rigid insulation as needed to achieve the desired R-value.

Thank you all for your time and responses. I am going to research your suggestions and come up with a plan that could work for my situation.

I think your ceiling is a ”wood plank ceiling,” not shiplap. Use that term to google for ideas to seal the gaps between the planks.