I'd need to have a specific description of the system, what exactly it can do, and how exactly it can modify itself, to give you a specific example of self-modification that contributes to the specific goal in a perverse way.
I can invent an example, but then you can just say "okay, I wouldn't use that specific system".
As an example: Imagine that you have a machine with two modules (whatever they are) called Module-A and Module-B. Module-A is only useful for solving Type-A problems. Module-B is only useful for solving Type-B problems. At this moment, you have a Type-A problem, and you ask the machine to solve it as cheaply as possible. The machine has no Type-B problem at the moment. So the machine decides to sell its Module-B on ebay, because it is not necessary now, and the gained money will reduce the total cost of solving your problem. This is short-sighted, because tomorrow you may need to solve a Type-B problem. But the machine does not predict your future wishes.
I can invent an example, but then you can just say "okay, I wouldn't use that specific system".
But can't you see, that's entirely the point!
If you design systems whereby the Scary Idea has no more than a vanishing likelihood of occurring, it no longer becomes an active concern. It's like saying "bridges won't survive earthquakes! you are crazy and irresponsible to build a bridge in an area with earthquakes!" And then I design a bridge that can survive earthquakes smaller than magnitude X, where X magnitude earthquakes have a likeliho...
In the spirit of "satisficers want to become maximisers" here is a somewhat weaker argument (growing out of a discussion with Daniel Dewey) that "tool AIs" would want to become agent AIs.
The argument is simple. Assume the tool AI is given the task of finding the best plan for achieving some goal. The plan must be realistic and remain within the resources of the AI's controller - energy, money, social power, etc. The best plans are the ones that use these resources in the most effective and economic way to achieve the goal.
And the AI's controller has one special type of resource, uniquely effective at what it does. Namely, the AI itself. It is smart, potentially powerful, and could self-improve and pull all the usual AI tricks. So the best plan a tool AI could come up with, for almost any goal, is "turn me into an agent AI with that goal." The smarter the AI, the better this plan is. Of course, the plan need not read literally like that - it could simply be a complicated plan that, as a side-effect, turns the tool AI into an agent. Or copy the AI's software into a agent design. Or it might just arrange things so that we always end up following the tool AIs advice and consult it often, which is an indirect way of making it into an agent. Depending on how we've programmed the tool AI's preferences, it might be motivated to mislead us about this aspect of its plan, concealing the secret goal of unleashing itself as an agent.
In any case, it does us good to realise that "make me into an agent" is what a tool AI would consider the best possible plan for many goals. So without a hint of agency, it's motivated to make us make it into a agent.