How do I change a block's ID?

[MoNkEyxJOSH]

Need to change the id for the class file 'BlockFirSapling' as it is conflicting with a mod I want to install. How would I change this ID or the one in the mod that it conflicts with?

[SimpleGuy]

My guess is by editing the source code itself. So you'll have to decompile, deobfuscate, edit the source file, then recompile, then reobfuscate.

[astocky]

Pretty much all mods have a config file in the \config\ folder. You just need to find out what mod uses the block in question and then edit the config file for it. The technics pack is pretty full though so you may need to use not enough items to find the block ids that are free.