I'm sure it can be done provided the replacement bars are not bent in three dimensions (eg drop handlebars). I replaced my Birdy bars with some Ergotec AHS bars. The only problem is that the width across these is more than the standard Birdy straight bars with the result that the end sticks up slightly higher than the rest of the bike when folded. A good starting place for planning a change is to fold your Birdy and see how the standard handlebars fit against the frest of the folded bike. Then try to imagine how your planned replacement bars would look when folded. If only folding and packing occasionally then you can slacken the bolts on the handlebar clamp to rotate the bars for a better fit. Or, in the worst case, remove the bars from the clamp and fix to the rest of the folded bars (I use Velcro cable ties for such jobs) to achieve the smallest size for packing.