Sounds like the company wants you to take on a second (demanding) job without a pay rise or promotion.
It's up to you if you take it, but to me it sounds like you aren't getting anything out of it.
It also sounds like you aren't qualified, so I would question if the company is completely oblivious about the tech industry, or are they setting you up to fail?
My position was a field technician and they wanted to move me into a hybrid role as a field tech & full stack developer.
Why pay 2 employees, when there's 1 sucker that will work twice as hard for half the money?
What do YOU want?
You need to discuss this with your employer. If you don't want the new position don't take it.
If you want it, then set the terms. - even if you see this as an opportunity to learn and do it without promotion, you should still set the terms and get it in writing.
This is the minimum I would ask for:
- You will take on this new position as a 12 month trial
- If things don't work out, you revert back to your old position
- If things are working out, the new position becomes official and you are officially promoted with a new title and a pay rise of no less than x%
You could also ask for back pay if they screw you (make you redundant) within x months of you being officially promoted, but you may need a legal professional to get it worded correctly, and I doubt they would agree to it.
They're now saying that I can become a senior full stack developer by developing a new program and some of the code from the old app to a newer app I'm creating in WPF.
This is worrying. It sounds like they are saying, if you submit and accept this unfair offer, we "might" give you a promotion in a few years, but we wont put this in writing so we can easily back out and not give you anything.
I don't find it possible to go from being a junior to senior level full stack developer in only a couple years
It depends what you mean by "senior level". In the last few years the "senior" label has become bit of joke in the industry. It's possible to get a senior label in 2-4 years.. but it will only be a meaningless label.
I've been doing this 12 years, and really it takes people at least 7 years to develop a senior level of skill for full-stack. 2 years is still very junior. At 4 years you will have developed a decent level of skill but you wont be a senior in anything other than title.
Without a team of experienced developers to learn from, in 2 years you wont have developed anything more than junior skills and bad habits.