My current boot partition size is 254 MB (checked with parted -l), is it big enough for LE 8.2.x?
afaik yes ? but do an backup, copy it off the box, then try an update
worst case you have to reinstall OE6x and import the backup again (OE has no protection against those kind of operations, LE offer that)
Upon update to LE the Skin also automatically updates?
yes, try to set it to the default skin before update to prevent problems (skin updates after first launch)
have modified versions of mediasources.xml, sources.xml and wakeonlan.xml > in general, those are also supported in LE?
yes, same as in OE
As I have OE running in NON-Windows OS, am I expecting to have hassle with the SMB / my working NAS mounts?
you may have to change some settings at the LE settings addon, depending if your NAS uses only the horrible outdated SMB1 or not - everything is possible but might need some setting changes (depends at versions you use)
Should I rename the old ones
just do nothing and it works