While most networks still use the Radio Bearer "Release with Rediredirect" method to switch from LTE to 3G when necessary, some networks have started using a real LTE to 3G packet handover procedure that significantly reduces the outage time of the data bearer. So far so good. The problem with this is that once a device is on the 3G layer there's no way for it today to get back to LTE until no data is transmitted anymore and the connection is put into Idle or Cell/URA-PCH state. This is especially problematic if a mobile device is used via tethering in combination with notebooks and other devices that send data all time as the switch back to LTE then never happens. Perhaps the time has come now to change this?
Before I go on explaining why the time might have come for this to change it's perhaps a good idea to have a quick look at the problem of a 3G to LTE handover. While active in UMTS, the mobile's transceiver is active all the time so it can't look on other channels and bands for a better radio technology. The only way to do this is for the network to schedule transmission gaps (the famous UMTS compressed mode) and to instruct the mobile device to look for LTE cells during those transmission and reception gaps. Obviously such a radio reconfiguration has a significant drawback: The data rate goes down. This is perhaps ok if an LTE signal is found but not very desirable if there is no LTE coverage to be found for some time. This is the reason why network operators have so far shied away from it. After all, 3G is quite a good technology for Internet access as well.
These days, LTE has become a lot better than UMTS, however, and when I look at network coverage maps there aren't a lot of places in many networks where 3G is deployed but LTE is not. In other words, if the unfortunate event occurs and the mobile is sent to 3G due to a lack of LTE network coverage, chances are very high that the user will be back in LTE coverage quite quickly. Therefore I think that with the LTE network coverage there is today it would make sense to think about 3G to LTE handovers.
P.S.: And it's not that changes from a slower RAT to a faster RAT while transferring data is unknown. This works great from GSM to UMTS for example. As GSM/GPRS uses timeslots, a mobile device has ample time even without network support to search for UMTS even while data is transferred. The same mechanism also works to switch from GPRS to LTE during a data transfer but so far only few mobile devices have implemented this. Fortunately first devices are now showing up that can do GPRS to LTE reselections during packet data transfer. So when I'm connected while being in a train I at least end up on LTE again if things get so bad for some time that my connectivity ends up on the GSM layer.