You are probably unable to enhance because the node is equipped in the V Matrix.
To unequip it:
- Open your skill window.
- Click on the big "V Matrix" at the top of the 5th job tab. This will open the V Matrix.
- Double-click on the old node that you don't want to use anymore. This will un-equip it and move it to the bottom row.
- Find the new tri-node that you want to use instead, and double-click it to equip it to the V Matrix.
- Close the V Matrix Window.
Now you should be able to talk to a Nodemaster NPC to enhance the new node with the old.
https://strategywiki.org/wiki/MapleStory/Formulas#Level_Difference
You do get a 20% base exp bonus for fighting monsters within one level of your own.
However, as MapleSapo pointed out, if you can only get in half as many kills per hour (either because of requiring 2 hits to kill them, or because the map's terrain or spawn are bad), then that's not worth it.
The game provides us with the Battle Analysis system, which among its other uses can also help find the most efficient place to train. Simply run Battle Analysis for 15 minutes at each map you're considering using, and see which one gives you the most exp.
If the addition of attack speed to Hero's Mastery skill happens in KMS, there is no way it will not come to GMS.
Instead of fighting against what can't be prevented, I recommend you (and fellow Heroes and Paladins) try to come up with suggestions of how GMS can alleviate the problem. For example, allowing people to convert their 1h Genesis weapon to 2h (keeping its potentials, bonus stats, and soul). Maybe also non-Genesis weapons? Or allowing the transfer of potential from shields to secondaries.
