You can upgrade just one partition.
Partition a physical disk using Disk Utility on Mac - Apple Support
That way you get to keep both. Note, the Startup Disk System Preference on Snow Leopard on Snow Leopard will not see Mac OS X 10.9 or later. Use the Option key to dual boot between 10.6 and 10.9 or later later.
An early 2009 model is only compatible with El Capitan 10.11.6, not Sierra or High Sierra.
While there is a hack to run Sierra or High Sierra, it is unsupported and may have trouble with updates.
The advantage of running El Capitan, which you can upgrade using the App Store of Snow Leopard and direct to your empty partition is that all web browsers except Safari have a current Java and Flash support.
Firefox current support only begins with 10.10, and the same with Chrome.
10.11.6 is also able to sync with iOS 12 devices.
There are several Firefox spinoff browsers such as Waterfox, Tenfourfox, Seamonkey, and there is also Opera, Omniweb, iCab, Camino and Roccat. Java will not be current for any of the 10.6 browsers. You need a minimum of 10.7.5 to have current Java.
Snow Leopard Server, which is hard to come by, is able to be virtualized on Parallels within Mac OS X 10.7 through 10.12 if Rosetta is a must, and you don't want to partition.
LibreOffice and Openoffice are excellent alternatives to Appleworks that don't cost as much as Microsoft Office or iWork in non-Rosetta Mac OS Xs. They won't handle the old databases, unless the databases are exported to CSV, and then their spreadsheet module works well.
