In most bunk bed designs, the lower bunk is the major stabilizing force in the system that is the bunk bed. By removing it, you render the loft bed unsafe. Most of your conversion work will revolve around reinforcing the frame to make sure the bed is safe after removing the lower bunk. Your effort will be rewarded by extra floor space in your child's room--more than 20 square feet for even a twin bed.
Setting Up the Loft
Remove the mattresses from both bunks.
Remove the lower bunk support struts or platform. In some designs, this may mean completely disassembling the bed and skipping steps during reassembly.
Insert the lower bunk support struts for both ends and the side that lays against the wall. Do not include the support platform, if any. If the platform and struts are one unit, remove the struts and install them if at all possible.
Measure the diagonal distance from the top corner of the end of the bed to the halfway point on the opposite post. Cut four support struts to match this length. It's best to use the same or similar material to construct these struts.
Attach two struts to each end. The top strut will run from the top corner to the halfway point on the opposite post. The second will run from that halfway point to the opposite bottom corner.
Measure the distance from the top corner of the side facing the wall to the opposite bottom corner. Cut two struts to match this length.
Attach the two struts, overlapping in an X shape, across the side of the bed that faces the wall.