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.
What do YOU want?
This is a new position, only take it if you are comfortable with it, but you need to set your terms. Even if you see this as an opportunity to learn and do it without promotion, you should still put your terms in writing.
This is the minimum I would ask for:
- You will take on this new position as a 12 month trial
- You will have a temporary new title of x
- If things don't work out, you revert back to your old position
- If things do 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%
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. I have a feeling 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. - this has happened to me before.
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". Do you mean in skill or by title?
In the last few years seniority labels have become bit of joke in the industry. It's possible to get a senior title in 2-4 years.. but in title only. To develop a senior level of skill takes a lot longer.
More importantly, if you are the only developer you wont have experienced colleagues to learn from. It's unlikely 2 years on your own will be enough to develop more than a few basic skills and bad habits.