I would urge caution in doing what for the most part seems to be a lengthening exercise, in that you risk losing part of the original point of the design, that it should be beginner friendly. Also remember that where possible a green should be visible from the tee, or without having to walk more than a few yards to see the basket. This makes it playable for newcomers.
Final cautious note, don't forget that you can always have alternate tees/layouts for club/tournament play, without changing the regular design.
That said...
Simple extensions: 1, 8, 11, 14, 17
1 - I can see it, but it would be horrible for tournament play with discs coming in on either 15 version
8 - Ok
11 - OK
14 - Why? It's the right length for the challenge at the moment. Make it longer and you risk turning it into 16, a pathetic chip through, a simple approach, dull 3.
17 - Extending par 3's into 4's by simple length is not usually a good plan, you need to plan a landing zone for the drives. But it might work, I'd have to play it to see.
Minor tweaks: 2, 3, 6, 7, 10 Edit: Rich has pointed out that these 5 are all actually the same, no change, so ignore this
2 - Already plays easiest on the course doesn't it? Isn't this now even easier?
3 - Ain't broke... Willing to give it a go, but the choice of lines and the deceptive simplicity of the current hole I think are perfect.
6 - Not sure how much this changes things? Looks ok.
7 - Ditto, is this because that sapling is getting too high? I can see it for that reason.
10 - Ditto again, what real difference does it make?
Total changes: 4, 5, 9, 12, 13, 15, 16, 18
4 - Looks like a traffic and blind-basket-lost-disc nightmare. Already has the highest lost disc count on the course in that hedge, this will make it 10 times worse.
5 - I think this looks really interesting, and has a ton of potential. It really brings the OB into play, and offers a high risk hyzer line coming back in at the last minute vs a trickier controlled anhyzer line.
9 - IMO the only reason to change this hole is for moving 4, and I think that's a mistake, obviously!
12 - This looks ok, though I suspect it's simply an easier hole, possibly with a blind green?
13 - Again, just easier?
15 - Same as 17 really, just adding length rarely produces a good hole, plus you have the added problem that (old) 18s basket is even more in play from further back, and you have speed-of-play issues for tourneys with 2 tees and a green on top of each other.
16 - Now everyone knows I have 'issues' with 16, so you'll probably expect me to like this... I don't

Looks to me like the same hole, but with an easier second shot.
18 - Removed all chance of a 2, without much increasing the chances of a 4. Plus you couldn't play 16/18 in this config in a tourney, way too dangerous.
So in summary, I can see a case for incorporating changes to
5, 7, 8, & 11 into the regular course.
12 & 13 look like interesting alternates, maybe not permanent changes, but good to mix things up every now and then.
1 is ok for club play, but a definite no for tourneys
Changes to
2, 3, 6, 10 & 17 I'm not sure add anything, but I'd have to play them a few times to see.
4, 9, 14, 15, 16, 18 look like complete no goes for me.