I was kind of under the impression the long lifespan of the joints was due to the lower viscosity oil fill. The bearings would never be dry as long as the boot was intact.

I would install the boots with the outer clamps tightened down. Pour the recomended oil through a turkey baster or small funnel into the small end of the boot with the axel standing vertically or close. Install the clamp on the small end. Then put the axles on the opposite side that you removed them from. It may not do any harm to drive some of that grease into the cleaned bearings either. I would still add the recommended oil if they were my axles.

Before installing the new boots clean the old lubricant out by repeated flushing. Also feel for grit in the cv joint that had the split boot. If no grit in whatever lubricant is still left you should be good to go. As long as there is currently no clunking at present.

You have no way of knowing if the axels are original or replacements. Reversing the axels can give you a lot of new unworn surfaces though.

Now there was some discussion about there being a left and right side axle assembly. Make sure your car was manufactured later than when they were in vogue. Seems there was a guidance spiral to keep holding oil back from a seal. I would want to research this before swapping axels side to side. It is in the archives or perhaps a more knowledgeable member could quote.