Enlarge cabinet opening for 30" range
The only way to know for sure if there is an option to open up the gap width across the counter top is to slide out the drop in unit and make measurements. It is typical that a cabinet installation will have opening compatible with a standard sized standalone range between the sides of the cabinets. However there is no guarantee that the installers did it that way. 

The first thing you can look for is to see if the counter top extends over the side walls in the opening by some amount. If it does than you may have the opportunity to trim the opening to a larger size. The following drawing shows what would be the ideal situation for you where the counter top overhangs the cabinet side walls by 0.25".



Success at trimming such counter top will depend highly upon skills, available tools and the makeup of the counter top materials. Be aware that a drop in unit hides the current edge of the cutout. When you slide in a new standalone unit the cut edges of the opening will be visible unless you apply some type of trim or re-surface the counter top. 

Another thing to be aware of is that on some cabinets the side walls of the cabinet may not be even with the front frame of the cabinet. Sometimes the front frame overhangs the side walls much as 1/4". In such case if the sidewalls are at 30" it would also be necessary to trim back the front rails. This can be done but if done in a sloppy manner can look pretty ugly along the front edges of the range.