I never tested this feature, but based on the understanding of the LTE system, please find my inputs as follows:
There is a message named as "ERAB MODIFICATION REQUEST " defined over the S1-AP interface. If you go through the message detail then there is parameter "E-RAB Level QoS Parameters " . This IE has one sub IE "QCI" which is mandatory and can be modified.
If QCI of an existing bearer can be modified then RLC mode can be changed since each QCI has associated packet loss error rate and packet delay budget rate. And RRM takes decision based on all these parameters.
In essence, it can be done.
I am still looking for others to respond to make sure that mine answer is correct.