Alright, so I downloaded Radix-Core (neccesary to run MCA) and MCA, and I tried to open my world where I'm building a city (which is progressed A LOT), and the game crashed just after loading the world. For reference, there are a few things you need to know:

1. I have disabled the Overwrite Villagers option, but I'd rather not have to do that.
2. Other mods I have installed are:
   a) Forge
   b) Optifine
   c) Decoration Mega Pack
   d) Custom NPCs
   e) World edit
3. I have also made a new world and opened my old one after doing Number 1. Both worked.
   And here is the crash report:
   ---- Minecraft Crash Report ----

WARNING: coremods are present:
Contact their authors BEFORE contacting forge

// Uh... Did I do that?

Time: 8/2/15 9:22 PM
Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(Unknown Source)
at java.util.HashMap$KeyIterator.next(Unknown Source)
at com.google.common.collect.AbstractMapBasedMultimap$WrappedCollection$WrappedIterator.next(AbstractMapBasedMultimap.java:486)
at net.minecraft.util.ClassInheritanceMultiMap$2.computeNext(SourceFile:90)
at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:143)
at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:138)
at net.minecraft.world.World.func_175650_b(World.java:3126)
at net.minecraft.world.chunk.Chunk.func_76631_c(Chunk.java:952)
at net.minecraftforge.common.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:46)
at net.minecraftforge.common.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:12)
at net.minecraftforge.common.util.AsynchronousExecutor.skipQueue(AsynchronousExecutor.java:344)
at net.minecraftforge.common.util.AsynchronousExecutor.getSkipQueue(AsynchronousExecutor.java:302)
at net.minecraftforge.common.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:12)
at net.minecraft.world.gen.ChunkProviderServer.loadChunk(ChunkProviderServer.java:117)
at net.minecraft.world.gen.ChunkProviderServer.func_73158_c(ChunkProviderServer.java:92)
at net.minecraft.server.MinecraftServer.func_71222_d(MinecraftServer.java:302)
at net.minecraft.server.integrated.IntegratedServer.func_71247_a(IntegratedServer.java:183)
at net.minecraft.server.integrated.IntegratedServer.func_71197_b(IntegratedServer.java:206)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:438)
at java.lang.Thread.run(Unknown Source)

A detailed walkthrough of the error, its code path and all known details is as follows:

-- System Details --
Details:
Minecraft Version: 1.8
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.8.0_31, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 85249392 bytes (81 MB) / 523501568 bytes (499 MB) up to 523501568 bytes (499 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v9.10 FML v8.0.99.99 Minecraft Forge 11.14.3.1499 Optifine OptiFine_1.8_HD_U_D5 8 mods loaded, 8 mods active
States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored
UCHIJAAAAAAA mcp{9.05} Minecraft Coder Pack
UCHIJAAAAAAA FML{8.0.99.99} Forge Mod Loader
UCHIJAAAAAAA Forge{11.14.3.1499} Minecraft Forge
UCHIJAAAAAAA customnpcs{1.8.0_beta} CustomNpcs
UCHIJAAAAAAA decorationmegapack{1.18} Decoration Mega Pack
UCHIJAAAAAAA RadixCore{2.0.3-1.8} RadixCore
UCHIJAAAAAAA MCA{5.0.6.1-1.8} Minecraft Comes Alive
UCHIJAAAAAAA worldedit{6.1} WorldEdit
Loaded coremods (and transformers):
GL info: ERROR RuntimeException: No OpenGL context found in the current thread.
Profiler Position: N/A (disabled)
Player Count: 0 / 8; []
Type: Integrated Server (map_client.txt)
Is Modded: Definitely; Client brand changed to 'fml,forge'

This may be a bug, issue, or a screw up on my part. I don't know where else to put this, so I apologize if this is NOT where this should go, but could someone please help me with my problem? I want the mod to overwrite villagers, but not to have to make a new world...

I'm not quite sure why this happens yet, but it is a known problem on existing worlds where MCA did not previously run.

Same problem as #467 and #460.

Should be fixed in 8d7ab24.