4 then 1...Tongue "Switching" in Highway 59

The standard is to have our lips over four holes, so yes, just move the tongue from blocking holes 1-3 to blocking 2-4.

I know this is important for you to break down, so I'll spend a half hour of my time and answer you as detailed as possible.

U-blocking on the harmonica is where air travels down the center of the tongue... commonly where the left side of the tongue blocks the hole to the left and the right side of the tongue blocks the hole to the right (most players will have their lips over three holes when doing this, e.g., when playing hole 2 your tongue would block holes 1 and 3). The tongue is either curled in a very evident "U" shape (this is also genetics at play here... not everyone can do this) or there is a much less of a prominent U-shape and more of a scoop or valley, where the air travels down the center (this is most commonly the case when players aren't aware that they're doing it).

This technique is not a bad technique, it just causes problems when using traditional tongue block techniques. When you perform a standard tongue block, your lips are over four holes (1-4 for example) and your tongue blocks holes 1-3, leaving the 4th hole to sound.

Slap
Achieved by starting with your tongue off of the harmonica, sounding a chord, all of which is LOWER than the final 4 draw note (except of course the 4 draw itself) and then slapping down with light pressure over holes 1-3. This creates a chordal introduction to the 4 draw, with all of the notes swooping up, like a glissando (gliss) to the higher note (4 draw)

Pull
Starts with the tongue on the harp, blocking all four holes (1-4) and then pulls off to sound the chord. The pull and slap techniques are commonly paired, making it important that the lips are over the fours holes (1-4) so that no mouth movement is necessary between the two techniques. In a U-block, the lips would be over holes 3-5, sounding the 4 draw in the center... then if you want to do a pull, you would need to move the lips over to holes 1-4. You can do a pull on holes 3-5, where the lips are for the U-block, but the sounding chord is a little thinner (and so is the slap if it's performed in this way)

Flutter
Same as the slap, just repeated

Octave
This is where the rubber meets the road. Having the lips over holes 1-4, you can slap (1-4 to 4), flutter (alternating 1-4 to 4), pull (block to 1-4), octave switch (4 to 1 to 4, etc.), or octave (1 and 4 together). In all actions the mouth is over four holes, IT NEVER NEEDS TO MOVE... VERY efficient. To perform the octave from the U-block requires a large movement, from 3-5, to 1-4. Keep in mind that these tongue block techniques are used in combinations at very fast speeds, which makes it important to stay in the 1-4 embouchure

Final Thoughts
In the end, it's not a crime to U-block... you can use all of the techniques I brought up. Your slaps and pulls will be a little thinner sound (it works for John Nemeth, and he sounds great) and you won't be able to use octaves as instinctively as in the tradition tongue blocking embouchure... you'll end up using octaves in a run and then return back to U-blocking... in other words you'll need to make a switch and then switch back, in the same way that the pucker player moves back and forth from puckering to tongue blocking

In the end...
Nobody cares which embouchure you use... they only care that you create great music. I don't care as well as to which one you CHOOSE, I only care that you are aware of the pros and cons. My job is to teach you how to most efficiently use technique in our world of masterful harmonica playing.

Best wishes on your journey through this... I know it's aggravating.