Sorry for the delay. Here are the rules I have written after the upgrade. There may be mistakes so any corrections and suggestions are welcome
# Old DB Homepage to new one. Replace dbname with your DB name in production
RewriteRule ^page/dbname /dbname [R=301,L]
# Old DB articles to new one
RewriteRule ^page/dbname/_/(.*) /dbname/$1 [L,R=301]
# Old forum links to new one.
RewriteRule ^forum?$ /forums [R=301,L]
RewriteRule ^forum/?$ /forums [R=301,L]