This is akin to building a wall around your garden after you've invited a bunch of friends over for some tea and cookies. If I thought for a second that some business was trying to lock me in by making it super painful to leave and come back, I'd leave and never come back. It's not like there's any shortage of project management web apps.
However, I highly doubt this is what they're doing.
They aren't building a wall, they are simply sending a signal that the value you are currently taking advantage of is higher than you initially paid for it - and we realize it.
I see nothing wrong with the way they have done it. If they forced you to increase your monthly spend as a current client, then that would be more akin to your scenario. But they aren't doing that.
A better analogy would be, they have a nice lush garden setup with fruit stalls everywhere. As one of the first 100 people into the garden, you get all you can eat for $10. However, for everybody after that you have to pay $20. If you know this, and leave the garden and want to come in later....well, whose fault is that?