Thank you. There are a couple of potential angles here. Both will be based on breach of contract although one is less likely than the other.
Initially, you would be arguing that the employer has breached your contract by not honouring the post which you were promised at the outset. However, if the only thing is a change in title and your pay remains the same, with no other financial losses, a claim would be difficult. That is because a breach of contract claim only really allows you to get compensation for any losses which you had suffered as a result of the breach.
The other angle is that whilst this is the same breach, your losses are based on you having to give up a stable job with a good title, for this. Therefore, had you know that you would be placed in the role they have put you in now, you would not have taken up the job. Therefore, there are no immediately identifiable losses, however you could try and pursue them for any potential losses you may incur in the future for having to leave this position and then seek alternative employment. So you may not necessarily wish to make the claim now as you will not know what such losses will be, but do it once you have a better idea of these. You have 6 years in which to claim so no immediate rush.
Of course, legal action should only be a last resort so in the first instance you should try and resolve these issues directly with the employer. Do not hesitate to threaten legal action in your negotiations with them if necessary.
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