If all other things had gone according to plan, we should have some new equipment arriving, including some new armor, weapon, and the new character class. We won’t know until we actually build it, but the old armor was fine for the job of the old party. The new armor is better, but just not quite as fancy. It may be worth the risk to bring one more character from the old one’s group that is, if they have any choice at all. It is, however, very possible they have none. My only warning right now is you should take all my advice with a giant grain of salt, and be very careful whether or not you choose to take the new character. If you are planning to buy the new new, I would strongly suggest you go with those. The high quality of the new equipment is, in my estimation, quite valuable.
You should be prepared to spend a lot of time deciding which equipment you want to bring with you. Every single one: your weapon, your armor, your spellcasting, your spellshelf, your armor. Every single item that you find, and you want to put on your character. There is no point to bringing anything you will not use.
Once your new character is ready for battle, you should return to find your allies assembled there. All along it is going to be a struggle to persuade them otherwise, and even then, you’ll still be tempted to take one more action. Many people will accept a second spell in exchange for your armor spell, and some will accept two spells if your main spell allows for it. If you want to, you can even use a bit of bribery here and there to gain one of your friends to join you for a free character. The problem is, most of them won’t want to take your side unless they like it to be your fight right away. It may be that they like to spend some time as a “support” character for a few more levels, but if you ask them at the same time, they will say they’ll want that time for more action. I think it is not a good idea at this stage to pick fights where you know your character is likely vulnerable.
Once you have them, and you are ready, you should talk to them, and try and convince them that it is worth their time to try and help you. If only just for one or two actions, you will lose. If you can convince them that their time is better spent on something else, and that, after all, they will be helping you along all your time in the new role. If they really want to wait and get their hands on some of your valuable items, you should be able to convince them that, to be quite honest, the only truly significant thing they can be getting from your character is a much better job with the current one. If that is true, I doubt they will want to wait to take the other side against you.
After that, it is just a matter of doing what you were doing before and setting your old party to full capacity. If you took all these steps, there should be a number of adventures or even campaigns that will be taking place during this time. If you chose to go for this route, you will not need to worry about the next campaign, because your level will be sufficient to bring everything you are going to need for it. It is not exactly a “bargain” to keep your level up and to use your resources to do everything you need, but there is no reason to go to a third level of a campaign just to have a few more hours of the old one, at least at first.
I hope that is clear about the issue of how to get level increases in the “new” classes. If you have found this useful, I sincerely hope you found it as useful a supplement to your Pathfinder rules as I did. I would be delighted to have others learn from the journey, and from the example of my experience with this.