I had to drill three new holes in my PCB to mount the new transistor...
Is that PCB a two-sided PCB? If so, your problem may lie in the statement quoted above. If, as is very common, you PCB utilizes traces on both sides of the PCB at that device location, it is likely that by drilling the holes oversized (assuming that you did just that) you drilled out the vias for that device location. If you drilled "new" holes -- i.e. not in the same locations -- and if the board has traces on both sides at that device location, you won't have vias there at all.
What this amounts to is no continuity between the solder side (lower side) of the PCB and the associated traces on the top side (component side) of the board. The vias are there to provide such continuity. Often they are simply made by plating the insides of the drilled holes, but they are also ofte made by inserting a tiny sleeve into the drilled hole, with the sleeve being flanged at one end prior to installation and then being swaged afterwards at the other end to provide a physical connection to the traces and to retain the sleeve in place.
The problem is easily overcome, space allowing, with a two-sided PCB by simply soldering the device on both sides of the board. Be sure to use a heatsink on each lead when soldering, but especially so when soldering above the board, as you are working considerably closer to the component body.
On the other hand, if your PCB is a multi-layer type -- IOW if there are embedded traces sandwiched between phenolic layers -- and if you have in fact drilled out the vias, you've got a much more difficult problem to solve. This is because it's much more difficult to regain connectivity with sandwiched trace layers than with exposed ones.
In cases such as this, when I need to replace a through-hole device with one having larger cross-section leads, I will generally try to avoid the likelihood of developing this problem by using a short length of bare solid hookup wire through the PCB, with those wire leads being soldered to the new device leads. IOW, I avoid drilling the PCB by using wire that will fit the existing holes. It's a little bit more involved, but it sure saves later headaches!