The main differences are the windows in the cab plus the doorway. It may be possible to fill and re-work them in the etches, but not sure I’d want to try. The position of boiler fittings also differs a bit IIRC, condensing gear etc. so more holes may be in the wrong place. Reference to the GERS John Gardner drawings will highlight all the issues that will exist. Shame the 67/69 aren’t available any more.
Get in contact with Jim at Connoisseur - he told me last year at Guildex that he was re-introducing his J69 at a price point below his J68. However, WT member Eastsidepilot lists some milled conversion parts on his website, if he still does them?