One of the key differences is the VS spar (edit: see below, actually the HS attach bars, not the VS). If it has been lightened by the original builder for an A model, you will have to replace it to go to TW. On a quickbuild, I believe none of the spars have been lightened, but you should cross-check with the plans to be certain. Otherwise, pretty straightforward to make the conversion - as noted, there are several threads here on VAF about doing so.