Upgrade XenForo 1.5.18 to XenForo 2.1.3 which includes chat software connected to it -- 7

Hello, >>THIS PROJECT MUST BE DONE THROUGH THE TEAMVIEWER<< >>IF YOU ARE NOT FAMILIAR WITH XENFORO PLEASE DO NOT BID<< Website has very old chat script and support for it does NOT exist anymore. Current chat script works on my Debian 8 (jessie) server, but I have working Debian 10 (buster) server where I transferred it and it does not work and it needs to be fixed. Chat script uses XenForo for authentication purposes but it does not work properly and must be fixed. Upgrade must be flawless and will need up to 1 week to make sure everything works after the upgrade before last milestone is released. Official warning from XenForo regarding upgrade from v1 to v2 [login to view URL] I do have access to older versions of XenForo if needed. Default language is not English and it must remain after the upgrade.