Problem Tekkit --> FTB server?

[DSet]

I have a problem, when i tried copying and pasting the world file from my previous Tekkit server to my new FTB Ultimate server, everything seemed to work until anyone tried to log on. As soon as thet happened, it said
"Disconnected by server"
"Server Closed"

I read a thread where someone said that i had to change Block IDs in feed the beast to the same as Tekkit, since the server saves every block by block ID.

If this is the only way of doing it, does anyone know about a way to change all FTB-IDs to the same as the tekkit IDs, at least for the blocks that existed in tekkit?

If there is another, more simple solution, please tell me![DOUBLEPOST=1373018026][/DOUBLEPOST]

 

[ThyCelestia]

The blocks that were in tekkit that were not in ftb would be turned into stone.

No idea why its giving a class not found error.

 

[cjm721]

Does your world file have the word Tekkit/teknic anywhere? RP2/GregTech are both very anti tekkit and if it shows up in the filepath it crashes. Also what mods you using on the server?