there are different parameters (vendor dependent, i believe) that can control this behaviour.
The MS has a certain physical capability to handle FR and/or HR. This is known by the BSC. If a call is in HR and the MS can support both FR and HR, there is no restriction in 3GPP : a HO can be made to a HR timeslot or a FR timeslot.
In alcatel, a MS in HR that does a HO will be sent to FR or HR depending on current load conditions in the target cell, and depending on TFO activation or not.