First, open a Terminal.
Status before:
$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *128.0 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 127.8 GB disk0s2
Get Lion Recovery Update v1.0 (431.91 MB)
Mount RecoveryHDUpdate.dmg
$ hdiutil attach ~/Downloads/RecoveryHDUpdate.dmg
Extract data from RecoveryHDUpdate.pkg
$ pkgutil --expand /Volumes/Mac\ OS\ X\ Lion\ Recovery\ HD\ Update/RecoveryHDUpdate.pkg ~/Desktop/RHD
Mount RecoveryHDMeta.dmg (in the RecoveryHDUpdate.pkg package)
$ hdiutil attach ~/Desktop/RHD/RecoveryHDUpdate.pkg/RecoveryHDMeta.dmg
Run tool that will finally resize current partition and create recovery
$ ~/Desktop/RHD/RecoveryHDUpdate.pkg/Scripts/Tools/dmtest ensureRecoveryPartition / /Volumes/Recovery\ HD\ Update/BaseSystem.dmg 0 0 /Volumes/Recovery\ HD\ Update/BaseSystem.chunklist
Creating recovery partition: finished
Status after:
$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *128.0 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_HFS Macintosh HD 127.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
This is done the same way with external or internal drives.
Start your Disk Utility (Applications -> Utilities -> Disk Utility). In the left pane, select the disk for which you wish to change the partition table (select the disk, not one of its partitions, disks are at the "top levels" in the tree view in the left pane).
In the right pane, select the "Partition" tab. You must repartition your drive in order to change to GUID, so in the "Partition Layout" dropbox select "1 partition" (or however many you want).
Click on the "Options..." button right below the list view showing your partitions and select "GUID partition table". Adjust your partitions as needed and hit "Apply".
Best Answer
No destruction will happen - This repartitioning you propose is possible to safely do.
The recovery partition won't modify itself (since it's in use) and will survive attempts to erase itself when it's in charge.
You also can resize things in general when booted from the main OS as long as files that are stored in the area being trimmed are mobile and not in-use or locked.
You have to enable a debug mode for Disk Utility to see and work on the Recovery HD so it's planned to be something you have to know a trick and go out of your way to tamper with. (Note: Debug mode is removed in 10.11 with the redesigned Disk Utility app)