The rogue pending task is likely a non-issue. If your jmx command went through without errors and you got the log message you can assume it worked. It won't show in the schema unless you run the ALTER statement which affects the whole cluster.
If you were switching from STCS then you wouldn't expect any recompaction, as SSTables will just be calculated to be in their relevant window. On the other hand switching from LCS would generate many compactions. The only way to really tell is to confirm SSTables are expiring as you would expect them to, or verify that new SSTables are not being compacted with older ones. You might need to wait for at least one time window to pass to check this.
