I've done it 2 ways
1) - I contacted the person who put them in & he charged €7 per blind to do this (to the best of my recollection, that was the cost - had them back after about 3 days)
2) I had a sewing machine on loan for a couple of days so I just cut off the bit affected & did the same size turn up, unscrewed the cord off old bit & took out the wooden rod......slide the rod into the new turn up & rescrewed the cord back on. Took about 5/10 mins per blind (these weren't very large blinds (about 1.5 mtrs & they were from my kids rooms, so not likily to come under too much scrutiny!!)