The PSR-E models don't have the Style Arranger feature as is found on the more expensive PSR-S and Tyros models, so there's no way to modify the onboard styles and no easy way to export them to external software for editing.
The simplest option would be to connect the keyboard to a computer and use a DAW to route all of the MIDI channels coming from the keyboard so they go right back to the keyboard. You can filter out any Bank Select and Program Change events from the incoming events, then add your own Bank Select and Program Change events to the outbound channels. You'll also need to go into the keyboard's Function menu and turn off the Local setting, turn on the KbdOut setting (which is probably already on, but make sure it is), and turn on the StyleOut setting. The result is that when you play on the keyboard it won't generate any sounds (because the Local Control is off), but the DAW will bounce everything (except the preset voices and kits) back to the keyboard to be played using the voices and kits that you selected in the DAW.
Another option is to record the style you want to change using the built-in Song Sequencer, making sure to use only the C Major 7 chord and to select all of the variations, letting the Song Sequencer record a full loop of each Main variation plus (if possible) a full measure of each Fill-In between variations. Then use the File Control menu to convert the internal song recording to a MIDI song file on a USB thumb drive via the Save SMF operation. Then you'll need to use a program such as Midi2Style to create a style file from the MIDI file, and change the voices and kits used to whatever ones you want to use. After you've saved the new style file, you can load it back into your keyboard as a User Style so you can try it out.