Previously it was not clear what you wanted to do, and you mention conversion of OH to H to get RNA to DNA. However, it make more sense now.
You were able to convert RNA to DNA by replacing U to T because aptamer is a sequence of nucleotides and not the structure. Thus, changes in sequences can easily be performed.
However, OH and H are the structural components and replacing them directly would not be a good idea. You got the structure file from PDB and I suggest you to get the files you need from there in stead of conversion by yourself.