I replaced my chain plates earlier this year with the boat in the water. I used the jib halyards to support the mast side to side while the shrouds were loose with no issues. The boat is in a protected harbor however. After 35 years there was light surface corrosion and the core at the chain plate deck holes were dry thankfully. We trimmed the old chain plates and use them as backing plates.

I have pictures on my blog: http://careandfeedingofnordicstarpartii.blogspot.com/