Alter a large mysql table 700 gb
A little late, but here we go...

If you can't add more disk space and can't promote a replica I think your only option (short of adding more disk space) is upgrading to MySQL 5.6 and using the online alter feature. The third-party tools that I know of all require a second copy of the table to be created before performing the swap. As you mentioned, the good news is that as per the Summary of Online Status for DDL Operations dropping a FK constraint should not require a full copy to be made.

I've had moderate success using the online alter feature but there are a couple caveats to be aware of:


There is no throttling of the operation and this can heavily load the server
Due to the above this can cause large replication spikes to replicas
Adding/dropping a column can cause primary key errors


The last caveat doesn't apply in this situation but it's a big gotcha.

I'd also be sure to carefully read the known limitations of the online alter feature too.