More than 2 decades ago, I started learning upright/double bass (fingerstyle) for a kroncong group in high school. And eventually, after listening folks like Paul McCartney, Bob Babbitt, John Entwistle etc I think I want to learn electric bass.
I knew how to play a few songs...didn't claim to be proficient on that instrument, though.
Is it difficult to switch from upright to electric?
Honestly, it's dead easy.
The transition the other way, electric to upright, is much harder.
I can't really add much more except anecdote. I've played both in my life, but started on electric & been predominantly electric my whole career. I spent a couple of years in my teens playing upright in a trad jazz band at school, then a brief stint in the city's youth jazz orchestra, so I did get very much used to it back then [40-some years ago].
A couple of years ago I had to re-acquaint myself with one after a very long break [decades]. The stretches are hard in comparison. Adjusting to shorter reaches, especially with frets for intonation, is a doddle ;) If you have to look at your hand for a week to hone in the muscle memory, that's about all it takes to get the hang of it.
Aside from that, just don't pluck it so hard as you would an upright. It will very quickly tell you if you attack it quite that fiercely… buzz & fret slap will be prime indicators. You also get a lot more tone variation from moving your pluck point an inch or two either way, forwards or backwards.
Turning the wrist action a few more degrees to get the same attack angle isn't too tough; as your left hand has dropped a long way, you seem to automatically rotate your right to match.
I agree with @Tetsujin’s assessment, that going from upright to electric is a fairly easy transition. They are both tuned the same and the basic principles of how both hands are used apply. Electric bass is physically less demanding than upright and does not require the attention to detail required for good intonation, unless you decide to play a fretless bass. I can pretty much guarantee that the first time you pick up an electric (if you haven’t done so already) you will be able to play things you can play on upright without much thought and effort. I would like to add a few more detailed comparisons that might be helpful to you.
The first thing is scale length. Upright basses are in the 42-44” range. Electric basses are usually 34” for standard and 30” for short scale basses. A standard scale length would probably be more comfortable to you.
The physical position is different by almost 90 degrees. I suggest using a strap because it is important to have the neck be higher then the body for more comfortable left and right hand positions and the strap will allow you to easily keep the bass at a good angle.
Setup is very important. I’m sure you know how easy or hard an upright can be to play depending on the string height, string type, string gauge and how well the fingerboard has been dressed. A poorly set up electric can also be difficult to play. Make sure your instrument is set up well. That involves a properly cut nut, string height, neck relief (with the truss rod) and a string type and gauge that works for you. Fortunately most of these things can be adjusted easily once you learn how but starting with a professional setup is not a bad idea. Regarding strings, flat wound strings will sound warmer and feel more like upright strings than round wounds but are more expensive and usually feel a little stiffer.
Another thing is left hand fingering. You can use the upright’s 1-2-4 method of fingering on electric bass, especially in the lower register but once you get to the upper frets you may want to start training the 3rd finger to work independently and use all 4 fingers. Some players use 4 fingers all across the neck. That is a personal choice.
In the lower registers, I would recommend keeping the 1-2-4 fingering. Because the standard 34" scale may tempt you to stretch your left hand fingers more than you should.
Playing 1 finger per fret (1-2-3-4) in the upper registers makes it possible to play with fewer hand shifts.
Also, if you are used to having all of the fingers down to fret a note, it should be possible to play faster to use each of your fingers individually.
If the string action is low, then it is very possible to accidently fret the next higher note by being too on top of the note position.
Playing with your right hand fingers perpendicular to the string, there may be some pain until you develop calluses. Playing with your fingers in the direction makes it much easier to play faster lines.
I would suggest getting a teacher, or watch a bunch of YouTube videos, to get some ideas on playing technique.
