I have had trouble with version 3.1.2 performing edits in the Geometric Data window. Fortunately, the HEC-RAS data files are pure text and can be opened in wordpad or notepad and edited with ease.
Open the file in a text editor and cut and paste the affected cross-sections from one reach to the other. (Your geometry file has the extension .g01.) You should save a copy with a .txt extension before hand to be safe.
Below is a sample geometry data file:
Geom Title=Creekview Village Flood Study
Program Version=3.12
Viewing Rectangle=-0.3753 , 1.8021 , 1.2184 , 0.1585
River Reach=Spring Creek ,1
Reach XY= 71
1.5116029 .7618675 1.5295269 .7885236
1.5341228 .7917408 1.5391782 .7935791
1.5511275 .8188565 1.550668 .8248311
1.5469912 .8303462 1.5502084 .8648153
1.5495491 .8695316 1.546075 .8718699
Rch Text X Y=0.8092508,0.7748471
Reverse River Text= 0
Type RM Length L Ch R = 1 ,15180.05,85,93.91,50
BEGIN DESCRIPTION:
151+80.05
END DESCRIPTION:
Node Last Edited Time=Jun/14/2005 16:52:39
#Sta/Elev= 7
0 427.05 33.38 405.57 83.4 405.57 85.87 408.85 210.36 412.3
307.07 416.88 357.48 420.71
#Mann= 3 , 0 , 0
0 .1 0 0 .045 0 85.87 .1 0
Bank Sta=0,85.87
Exp/Cntr=0.3,0.1
Type RM Length L Ch R = 1 ,15086.14,120,90.16,60
BEGIN DESCRIPTION:
150+86.14
END DESCRIPTION:
Node Last Edited Time=Jun/14/2005 16:52:39
#Sta/Elev= 7
0 424.4 37.46 405.47 87.82 405.47 90.65 408.98 168 411.26
273.38 416 330.15 421.02
#Mann= 3 , 0 , 0
0 .1 0 0 .045 0 90.65 .1 0
Bank Sta=0,90.65
Exp/Cntr=0.3,0.1
The first part describes the coordinates of the points on your reach in the Geometric Data window. Below that is information on two cross-sections. When you move the cross-sections to the correct reach (and be careful of the order!) then you'll have to change the first line to indicate the new reach number:
Type RM Length L Ch R = 1 ,15086.14,120,90.16,60
would become
Type RM Length L Ch R = 2 ,15086.14,120,90.16,60
Once you've made the changes, open the cross-section window to check it worked before deleting the old copy!
Good luck!