IIRC in version AMXx v1.50 or one of the other some debugging stuff was change. You only had to recompile the plugin if you needed to debug or you would not be able to run the plugin in debug without recompiling the plugin. The plugin would run fine except for that. It not that hard to recompile the plugin for your current version of AMXx
edit.
If the plugin was compiled prior to AMXx 1.50 IIRC.