Unicode content

If we convert a forum Mysql DB from latin1_swedish_ci to utf8_unicode_ci we have to convert exisiting posts too? Or can we run the existing posts in utf8_unicode_ci tables while new posts added as utf8_unicode_ci ?
How this will effect performance and what you recommend?

i am going to use vbseo in this forum and it has around 8 million posts.

If the table is converted, the post content should be converted as well. Mix-match will likely result in errors.

I HIGHLY suggest exporting to a temp db and playing with it before even attempting something like this on a live db. Hiring a freelancer DBA may be advisable as well.