It would be highly ill advised to replace just the front glass on your device. Your water proofing is going to be compromised if you don't do not reassemble it correctly. Most of all the prospect of damaging your led is very high; not only from separating the glass but the heat that you will be applying to melt the LOCA that fuses the glass and led.
You should indeed replace the entire front assembly to minimize risk. Think about what you'll have to spend if you brake the led during the repair process. You'll have to buy a new front assembly anyway if your repair fails.
If you insist on doing the front glass repair keep these tips in mind:
When your heating the screen keep in mind that heat will flow to the coolest parts of the glass and disapate so try to heat evenly.
Make sure you pre heat the screen at a lower temperature and heat at a slow pace gradually raising the temp. Pre heating is always good; gives the molecules time to do their thing and allows the matireal to hold heat better.
Your not going to want to use fishing line because it's plastic and may melt in the heat. Use thin metal line to do the work.
Once your under the glass with the line make sure you keep the temp constant as you progress. Applying heat every half an inch or so.
The led is paper thin so make sure you use a light touch when conducting this repair. Too much pressure or moving too fast can cause the led to crack so slow and steady for the win.
The main thing: slow is smooth, smooth is fast... And inexpensive.
Good luck to you bud.