I run a fairly large board with over a million posts. I was happy to migrate from VB4 to Xenforo about 4 years ago and have a number of free and premium add-ons.
I've been reticent to upgrade because I don't have a lot of time and am nervous about what happens with all the add-ons. I'm pretty sure most of the add-ons I really care about will be available in 2.x form but I have a more basic question.
What can I expect during the upgrade process? Do I need to disable all my add-ons or will the upgrade process do that for me? Is it possible that some of the add-ons will continue to work even before I upgrade them?
I also have the premium add-ons like Enhanced Serarch, Media Add On, etc. Will those break during upgrade and only start working after I upgrade them?
What does it "look like" to upgrade a legacy add-on? Is there an upgrade tool within XF 2?
I had a very long look at this. was in the exact same situation. board with 1,000,000 stayed on xen 1.5 due to addon problems upgrading. I have a difference though most likely. I have posts all the way back to 2008. So 12 years of posts.
Things I have found. (I upgraded 6 days ago)
Research your plugins. MOST of them will not upgade at all. even the ones that show you using them in 1.x and have a 2.x version, will still not upgrade and unless you manually spend a LOT of time remapping you are going to lose that data no matter what you do.
*Sidenote, LEARN CLI if you don't know it already. using this (command line interface) you can do many tasks without having to go through the painstaking work of rebuilding everything through the browser and it takes 90% less time to do things like install/upgrade addons and most tasks you do that take rebuilding.
Our process went extremely smooth but it's because I finally resigned to losing the data. For instance, we had a rep system for 3 years that i had to dump. In fact the ONLY addon I had that upgraded was sportsbook and it did so beautifully. I lost 100% of the data for everything else.
Sooner or later you're going to have to bite the bullet and the sooner you do so the better. This inclues your styles. However, you have an option here.
DO NOT remove your 1.x themes and they will come over during the upgrade, but they will be broken. There is no other way to get those styles into your 2.x install. I did this and after a few hours reworking styles like the 'digi' theme I got pretty close. it's not hard, just tedious.
Now, the big thing, your addons. I set up a sandbox. I removed ALL of the addons that did not upgrade in the old site before upgrading. including the xenforo addons (don't know if this is required but I did it). There's a reason for this. if you don't not only disable, but UNINSTALL them, you will have all the data in your database that's no longer needed. in other words, junk data. It's not required to remove this but it's pointless to have it. I opted to remove all plugins except sportsbook so I would have a clean database.
on to styles. Remember, removing styles does not remove the folders for them. look in your /styles directory in SSH and you'll see even if you remove styles the folders and files are still there. This is handy if you're going to rebuild them because all the images files will still be there too.
Because I did a massive cleansing of our database the upgrade was smooth as silk and very fast. But we lost a ton of addons.
One other thing to think about. Most of those addons that were free in 1.x will now be paid addons. there are very few free ones compared to my experience with 1.x. just be prepared.
Any other questions let me know and I'll try to help.
Results: My users are only bummed about the reputations. but to me I think rep is stupid. it's good to clean it out after 3 years and have them all start fresh. the good ones will still rise to the top. unless they've changed they'll still be top people. They love the new forum now, and other than rep, seem to be happy. I'm definitely happy as the server owner.
I installed a rep system for 2.x but haven't started using it yet.
Final note: If you've ever done PHP work you will know this already. I followed these guys. All the way back to when they started vBulletin because it was the cleanest code I had ever seen anyone write.
They sold vBulletin and I didn't know it. I spent $500 on the new vBulletin and it was a scam. the people that bought vBulletin were denying all chargebacks at PayPal and the conversions were terrible. They re-wrote the entire code base and it was junk.
I was one of the lucky ones, i found out the original group had left, but i DID manage to get our $500 back. I bought Xen when i realized it was the same group. ever since that day I can confirm it sure seems to be the original group because the XenForo code is clean as a whistle again and we love it.
Whatever you do, IMO cater it to staying with XenForo. your addons and forum will adjust to anything you lose doing the upgrade, and you can pick up most addons for 2x to rebuild your forum with the same features.