All relevant product information can be found on the Analox Sensor Technology website, take a look at the O2NE+ and Safe-Ox+ product pages.
It should be possible to connect an older Safe-Ox repeater to a Safe-Ox+ unit or an older O2NE repeater to a newer O2NE+ unit as the repeaters are electronically similar throughout the O2NE+ and Safe-Ox+ version history.
It is important to ensure that any differences in device labelling won't lead to ambiguity and confusion. Also, a functions test should be carried out after installation to check that the repeater and main unit remain fully functional.