I have not been able to figure out any changes in MAC layer. If there doesn't occur any change then please give the reason
Not sure of the type of handover you are talking here. In general, Inter-RAT handover of LTE to UMTS does not require any change in MAC layer as the PDCP layer takes care of the packet delivery.
The handover procedure shall be similar to that of Intra-LTE mobility banked upon S1-based handover based procedures.
SGW plays the role of anchor in case of handover with other 3GPP technologies like UMTS or GPRS. In case of mobility from S1 mode to Iu mode, as soon as the source eNB sends a handover required message to source MME, it will do a forward relocation request to Target SGSN.
The target SGSN does the mapping of the EPS bearers to PDP contexts. It also maps the EPS bearer QoS to that of Rel99 QoS parameters and triggers a Relocation request message to Target RNC.
The target RNC allocates the resources and returns the applicable parameters to the target SGSN in the message Relocation Request Acknowledge message.
Based on the Forward relocation response from Tgt SGSN to MME, it can establish path for either an indirect forwarding or direct forwarding.
Once the Handover complete is indicated by UE, If it is direct forwarding, source eNB will perform direct forwarding of DL data to Target RNC else if it is indirect forwarding, source eNB will forward the DL data to SGW and via Tgt SGSN to Target RNC.
Seamless handover does not use a STATUS TRANSFER msg and hence it should be fine. However, incase of lossless handover, the PDCP status preservation shall not be present(context shall not be continued) as STATUS TRANSFER message was introduced in LTE and not supported in UMTS.