If you approach this cartridge switch with care and caution you should do well. Take your time and enjoy the process. Cartridge setup and alignment is a good skill to have. I'll emphasize to be especially aware of the cantilever and stylus while you're installing and aligning the cartridge. Keep the tonearm stabilized while working on the cartridge. And those headshell wires are VERY fragile. Following are a few web site suggestions (you may already know some of them) to help in understanding tonearm geometry and cartridge alignment and setting up a cartridge...
For specifications on turntables, tonearms, and cartridges, and the alignment geometries that might work with your setup, you won't find a better place than the Vinyl Engine databases.
Here's the Rega RB300:http://www.vinylengine.com/tonearm_database.php?make=Rega&mdl=RB300&sort=2&eflo=&efhi=&ascdesc=ASC&mdlo=&mdhi=&ohlo=&ohhi=&search=search&amlo=&amhi=&cw=&mp=
With the information provided for effective length, overhang (O/H), and offset angle (O/S) you can plug those specs into the alignment calculator to see how the RB300 tonearm geometry compares with the typical geometries:http://www.vinylengine.com/tonearm_alignment_comparator.php?m_el=237&m_oh=15&m_oa=22&compare=d&submit=calculate
As shown in the link, the Rega RB300 tonearm has a somewhat unique geometry, designed for a combination of Stevenson/Lofgren A (Baerwald) null points. But if you can slide the new cartridge forward enough on the headshell (from 15mm to ~16.3mm overhang), and twist the cartridge enough to go from 22d to ~22.5d offset angle, you should be able to align to the Lofgren A (Baerwald) null points.
If you're fairly serious about cartridge alignment but don't want to spend much money, I highly recommend downloading and using Conrad Hoffman's "TemplateGen" protractor, available here:http://www.conradhoffman.com/TemplateGen.zip
Unzip the files and read the README.TXT file. It has a very good introduction to cartridge alignment and step by step instructions on how to use the software and printing out a protractor template. It's an excellent protractor; and free!
There are many articles on cartridge setup so spend some time researching and reading up on the topic. Michael Fremer's DVD on vinyl setup is an excellent resource. For more information on cartridge setup here's an article worth looking at:http://www.audiophilia.com/features/cartridge_setup.htm
But don't get too worried about the recommendations for tools, equipment, test records, etc.; try to focus on the basics and you'll end up with a very good alignment. Then spend some time listening and experimenting with adjustments in your setup. You'll progress from very good to excellent with practise.
One thing I would recommend purchasing though, is a good tracking force scale. Shure makes a simple beam scale and you can find inexpensive digital scales that work well. Start with a VTF that's in the middle or a bit on the high side of the recommended range. With a brand new cartridge the suspension hasn't settled in yet and you can lighten up on the VTF as it does.
Have fun! And keep asking questions. You'll probably end up with more advice than you can use but it's interesting to see all the perspectives on the topic.