As of Biomes O Plenty 0.5.2, changes have been made so that the number of block ids used by the mod are drastically reduced. However, as a side-effect of these changes, existing worlds will no longer work.
However, thanks to a little Java application I (Adubbz) whipped up, as well as the lovely folks who made Midas gold, this should hopefully be easy.
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Backup your world
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Grab a copy of the Converter Patch Generator from here:
https://github.com/ted80/BiomesOPlenty/tree/master/BOP%20Converter%20Patch%20Generator (Click on the jar file and then 'View Raw' to download it)
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Grab a copy of Midas Gold from here:
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Extract Midas Gold.
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Launch the Converter Patch Generator.
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Click on "Import Config" and select your old Biomes O Plenty config file.
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Click on "Output Patch" and choose where you want the patch file to be outputted to.
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Hit "Start".
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Launch the Midas Gold jar file.
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Click on "Add Savegame" and select the folder of the world you want converted.
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Click on "Load Patch File" and select the patch file created by the Converter Patch Generator.
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Hit start!
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You're done :)