You don't have to physically take the button out to replace the LED. The power LED pulls easily out of the holder it's in - there's just a tiny bit of hot glue holding it in.
I've replaced the blue LED on my Define Mini and Define XL with green ones, and glued a red LED to the side of the clear plastic surround to act as a HDD LED. Works perfectly - the plastic glows green and when the HDD is accessed it flashes red. I just hot-glued the red LED to the side of the clear plastic, beside the green one.
Both LEDs are normal 3mm items.
Workstation: i5-4690K@4.7GHz, Asus Z97I-Plus, 8GB Kingston DDR3-1600, Crucial M500 480GB, WD1003FZEX, Win7
Server: i3-4350, AsRock E3C224D2I, 16GB ECC, Intel 520 180GB, 2x Toshiba 5TB, 2xWD50EFRX, Server 2012R2, Stablebit Drivepool
HTPC1: TranquilPC ABEL H2, i3, 4GB DDR3L-12800, Crucial M4 128GB mSATA, Win7
HTPC2: Akasa Euler, DH61AG, Pentium G620, 4GB DDR3L-10666, Crucial M4 64GB mSATA, Win7