Also, I know there are many Vbulletin members either considering moving to IPB 3 or are in the process of doing so, but want to make the move as painless as possible. Especially since they may be adjusted to their current forum software. A few suggestions that always helped me. (You'll always get some people that grumble no matter what. They too will go quiet after awhile.)
1. Promote the things IPB 3 has that VB 3 and even VB 4 do not have or do not yet have. Such as a more community friendly member profile. Or improved PM and User CP options. Basically, you know your members. Push the things that IPB has by default that you know your forum members will like, but may be afraid of losing with the switch. Then promote the positives of that. Simple, yet effective.
2. Also let them know they will NOT lose their posts, pm's, threads, attachments, etc.. And if they love the skins they have, show them some IPB 3 ones you know they'll probably love even more.
3. While some are always against change, most from what I have seen usually adjust and end up liking some of the new features better. Such as IPB 3's improved attachment system.
4. If they ask if a certain mod or feature will be brought over that you either know or think is not yet available for IPB 3, let them know that if it isn't, you'll put in for a request for someone to make that mod. Many times this will pacify them.
5. If they ask what the pro's vs. con's are, let them know you will leave it up to them. But that you have come to find that IPB 3 is a far cry from previous IPB versions they may be aware of and is in no way close to Invisionfree forums. That it is above them in every way and has features that at this point it appears not even Vbulletin 4 will yet have.
6. And for the mods and admins, let them know that IPB 3 has many of the same banning (and even improved) features that VB has.
Long story short, push the pro's. (IMO there are quite a few.) Focus on hyping them up. That will make the transition easier. What I always did, was found an IPB 3 forum that had a skin I knew they would like. I would then have them visit that forum to get an idea of what IPB 3 is like. (That included pointing them here.) By the time it was said and done, most were hyped for it and things went great. In fact they never wanted me to go back to Vbulletin again after that. They liked IPB 3 way too much. To them VB seemed like a step down. (Which IMO it is anyways. But I digress.) Once they start playing with their profiles (which is what I pushed them to do), they loved it and started spamming each others comments. It was great. It wasn't some bland profile anymore. It was a robust social network for them. And the longer they stay on your forum, the better.
IPB 3 is just as robust if not more so than VBulletin 3 and even what's known of Vbulletin 4. Does Vbulletin 3 or 4 have a built in virus scanner? Does it have community friendly profiles? Simple answer, no. And that's an EXTREMELY short list.
In the end it's up to you as to what you want to do for you and your members. But I'm confident most of them will enjoy what IPb 3 has to offer. :)
