it probably could be done for posts containing a single table:
Update Posts
Set body = left(body, charindex(body, '{table}')) + (sql code to transform the table: a bunch of replaces on the string between the two tags) + right(body, len(body)-charindex(body, '{/table}'))
where body like '%{table}%'
--Confirm that there is only one table tag:
and charindex(body, '{table}', charindex(body, '{table}')+1)=0
But it could get very messy for more than one table in the same post: SQL doesn't really provide the kind of iterating tools and functions you would need.
(Qwerty, if you want me to bore your ear off with SQL logic, drop me a PM)
Grey Wolf, most definitely NOT offering advice on how to approach the change - just explaining the dangers of doing so with SQL