I don't have a Solaris system to test on: this should work:
sed -E 's/^(.{233})R/1x/' file
That does not modify the file. To save a backup copy and modify the file in-place:
sed -i .bak -E '...' file
Use -i ''
if you don't want a backup.
Reiterating: I don't have Solaris, and these option may not work with the Solaris sed.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…