The conversion of your pictures to a higher resolution is possible. Changing a resolution for a file is possible with any image editor available that has the Resize feature but going from a small resolution to a bigger one will drastically cut down the quality of your images. The backward process is possible with a minimum loss of image quality but what are you asking involves quality loss on all fronts.
For example if you have a picture with 800x600 resolution and you want to make it bigger, let's say 1024x768 then the quality will slowly decrease because you are enlarging its size and basically it will be somewhat ok because it's an appropiate resolution and the quality is acceptable but if you reverse the process, the quality will barely change.
If you want, you can use XnView to resize your image. Download and install the software and run it from your desktop then load the image and press Shift+S and a pop-up will appear where you can enter the desired resolution.
I have attached an image where you can visualize what I explained: