Leo,
Cast bullet loads are waaay below SAAMI specs for pressure. Therefore substituting a jacketed bullet should present no problems.
if using H4895, and you start at their 60% rule, since it has been tried and tested by ADI who gave them the information, there should be zero issues. Same can apply to using IMR 4895 instead. All of these KaBooms one can hear about, happened with much slower powders being reduced, such as 4350, 4831 etc. Even P.O. Ackley tried to duplicate the situation in the lab, and couldn't, so he couldn't conclude why it happened.
Powders like 4198, can be reduced lower than the 60% rule with zero issues....I refer to Ingwe's Bunny Loads, as they are called on the campfire here, 12.5 grains of 4198 with a 50 grain bullet in a 223 case.
Using one's head, instead of letting old wives tales rule your thinking when at the load bench, goes a long way.
Most naythsayers will admit, they haven't worked with downloading cartridges much, if at all.... and then justify their lack of hands on experience, with the excuse "they know better". Apply that logic to the subject of driving a car, and you'll see how ridiculous that is.
Cast bullet manuals have loads that have been tested, so one is pretty safe with those. The most important issue in safety is one's loading techniques at the load bench. Its no different than when loading handgun cartridges, yet somehow rifle handloaders often think it is a case of being a mad scientist.
