I am developing a software project in Java/Swing licensed under GPL v3. Later, I want to create a Android application which uses the algorithms of the Java/Swing application. This Android app will be a commercial product (sold on Google Play Store).
Is this a problem, when I use my OWN GPL code in a commercial SW developed by me?
If you are the sole copyright holder (i.e., the owner), you can do anything you want with the code, including doing a derivative version of the code where the only change is to the license. Licenses are just descriptions of the conditions placed by the owner(s) on the non-owning users of the code. They do not constrain the owner.
When there is multiple ownership, things get more complex (formally, all copyright holders have to agree in order to change the license). There's a gray area about what sort of contribution would be required by someone for them to be a copyright holder; its almost certainly not done by mechanical count of lines modified as a substantive contribution could be very short and a non-substantive one very long (e.g., converting all the indentation to tabs or spaces). We can't assess the extent to which this applies in your situation, except to point out that someone else downloading and using the code doesn't obligate you to grant them ownership rights.
If you write all of it yourself, you don't need to pay much attention at all to the complexity in the previous paragraph. You can just go ahead and do what you want to do.
A separate point is if you are working for a company that is the owner of the code. In that case, it's the company's decision and you're just acting on the company's behalf. It's no more complex than before provided the company is the sole owner of the code.
No, there's no problem as you own the code (and the copyright). You can do whatever you'd like with it. You could also allow other people to do the same for an additional fee if you provide them with a copy under a different license.
In the past, I've changed my GPL code to do something that a client wanted, and then provided them a copy under a two clause BSD license (specifically so they could link to it in a proprietary application).
There is one caveat, if you accept more than trivial patches from people offered to you under the terms of the GPL 3, you'll need to get them to sign a copyright assignment (so that you retain full ownership of the copyright). If this is just a one person project, you don't have to worry about that.
Other answers have already said that you can, as the copyright holder, change the license of your own code at any point. I will not insist on that.
I just wanted to add a detail that nobody else has mentioned: you can sell GPL code on Google Play in a commercial product. Nothing in the GPL prevents your Android GPL application from being a commercial product that you charge money for.
The only requirement for distributing GPL software is that you must distribute the source: distributing the binaries is optional, and charging is optional, but there is nothing wrong or illegal on distributing binaries and sources for a fee (i.e. selling GPL software).
Is this a problem, when I use my OWN GPL code in a commercial SW developed by me?
No, why do you ask? Because you were so free to have released it once under GNU GPL? You don't marry the GPL, so as long as you are the author and copyright owner, you can change the license for each release you do. You are in control.
To release a non-free program is always ethically tainted, but legally there is no obstacle to your doing this. If you are the copyright holder for the code, you can release it under various different non-exclusive licenses at various times.
However, if your code has other authors and copyright holder, too (e.g. being a derivative work), this depends. You have not said so, so this is just to give the counter-example.
