Wanting to integrate with existing user base and trying to decide if I should convert existing users to YAF or just keep the standard provider.

and If there is no advantage, why is there a YAF membership provider in the first place?

For most forums, this is not important. Otherwise, it supports YAF.NET specific features such as boards, better. Also, there are a couple optimizations which improve speed.

The ASP.NET providers work fine.

Install the forum with an empty membership database to avoid conflict with already present users and then copy users over afterwards or will the installation be able to handle the existing user base?

Anything that needs to be updated in the YAF tables? Will dbo.YAF_User automatically be populated when the user visits the forum for the first time?

Thanks!!

It's automatically populated from any Membership provider.

If you have a choice, always use Yaf providers. They're fast and related to each other at tables level and the data is cached too. They keep data in tables and you can easily control the data integrity.

I imagine YAF_User is normally populated at signup, but since this hasn't happened in my case, what happens when one of my users visit the forum while logged in? Will it create an entry in YAF_User or throw an error?

Your imaigination plays tricks on you :-d

They appear in the yaf_User after a first forum page visit in every case if you've made the rest OK.