You can downgrade, but you should be aware of posible side effects that you may not wellcome.

Alternatively you can upgrade to your "custom" version by branding it to a number higher than 29.1: hex-edit files lng_rel.tbl and sw_rel.tbl in \V_2\RR\0101\BMWC01S. The first four bytes "0910" indicate the software version (29.10). You can simply edit the fourth byte to brand your custom version as 29.11.

These are binary files. Do not use but hex-editors with them.