Upgrade SG to XG with a 3 year total Protect License and 2,5 years left

This KBA explains everything in detail.

The License will be migrated in the same duration left. 

https://community.sophos.com/kb/en-us/124588

This KBA is more about the License Part:

https://community.sophos.com/kb/en-us/122768

Thanks for that - thats great!!!

if i want to extend the license later, can I take a XG license then?

You will bought a special license but most likely, you will add a hardware refresh at this time. But yes, it is possible. You do no not buy a SG License, instead XG. 

Thanks for your Answere.

I said: "but most likely, you will add a hardware refresh at this time"

Is this because its normal to update or is the SG Hardware not as good as the XG Hardware?

In one case for example it's a SG115 Rev1 for 10 Users; But only 6 of them use internet connection from office behind that Firewall. The other 4 are mobile workers which mostly use the Exchange with their mobile phones.

At this time it is not planed to change the Hardware at the end of the license time: I think at least with SG we would not upgrade it. Do you think that will be with a Upgrade to XG more require?

The hardware should be the same. 

XG115 REV1 has the same hardware like SG115 REV1. 

Simply the Revision upgrade is a good way to upgrade the hardware. For example, REV3 (because REV2 has no "real" difference in it), you get a SFP Port, VGA was replaced by HDMI etc.  

I do not know, where you came from, but here in Germany, you buy something and use something like "https://en.wikipedia.org/wiki/Depreciation". I am not quite sure, if this is the correct phrase but most likely our customers can buy every three year something new. 

So they will buy a XG instead of SG, because they run already SFOS (XG) and there is no reason to buy a new SG and reimage it.