How common lying is probably varies from place to place, but in my own experience exaggeration is very common, and lying is not that rare either. A few years ago I was promised some capable developers in my team but the small company where I worked as the lead developer ended up hiring the cheapest labor available to save costs; further back when I worked for a software giant, they employed me as a developer but the actual job was testing.
Some managers / business owners do not understand that not meeting promises and letting developers do what they love sabotage morale, which in turn may significantly adversely affect work quality, because software development is creative work and requires a good mood.
OK, enough ranting. I'd like to share my two cents in helping detect whether the technology migration from advertisements are genuine.
To reduce the chance of being fooled by a potential employer, I myself now use the following tactics wherever possible during job hunting.
(1) Ask for a trial day (or a couple of days) before signing up. Explain to the employer that hiring the wrong person can unnecessarily cost a fortune (if a developer formally starts and then quits after finding out the facts, the employer would have to re-advertise, pay another headhunting fee if they use an agent, go through the interview process again, and not to mention the lost opportunities and revenue). Some employers will agree, because they would like to see how good the developer is at real work anyway. During the trial a developer would have the perfect opportunity to see some code, how decisions are made, and product backlog / road map (could be strong indication of migration timeframe and strategies), if the employer allows.
(2) If trial day is impossible, ask to meet the senior developers. During the conversation, try one's best to ask in-depth questions involving the advertised technologies. If the seniors are knowledgeable in that area, or better yet, talk about some recent experience on applying the knowledge, the company is likely to be on the right track towards the advertised technologies. If the seniors do not know much about the technologies (they just thought it is cool to migrate to the new technologies but have not done much research yet, they will probably be busy maintaining the legacy and forget about the migration in a few months), or sneer when talking about the migration in question (their developers may be wanting the new technologies for various reasons, but probably have had difficulties convincing the company to actually start it), the migration is not likely to happen soon.
(3) If the state allows public access to previous legal decisions, search for employment relationship decisions involving the company. Most companies have clean records, but if the potential employer has lost in more than a couple of employment cases, their history of integrity and honesty should be questioned, so should the genuinity of the advertisements.
(4) If all else fail and one absolutely wants to work for the potential employer knowing the risks of not having the promises met, at least apply game theory during negotiation. Make it clear to the employer that being interested in learning and applying the advertised technologies is the biggest factor in deciding to take the job, the candidate is firm to quit the job if the promises are broken, and insist to have the migration work as part of employee performance review. The potential employer will think twice before deciding to lie, knowing that the candidate is not someone who will patiently wait for three years before the actual migration.
I know as a developer we should not focus too much on certain technologies. Instead we should learn more about a company's product, business and process, using the technologies just as a tool. However, I do believe developers should have access to true information and be able to choose the type of work (overall labor market is a free market in most regions). A company that cannot keep promises (with reasonable circumstances), or is unwilling to invest (given that it has the capacity, and benefits can be clearly seen), or wants to mislead potential employees (instead of treating them like a future partner / comrade), will unlikely to flourish and thus working there can be a waste of time if one wants to be successful.