Hello Steve,
NX5500E, as part of Express portfolio, is also end of sales since October 30th, 2017.
Since WiNG 5.9.2 you are able to convert Express to non-Express (Enterprise if you wish) WiNG.
Then you will be able to expand your deployment in same manner.
In case you wish to stay with Express SKU, you shall NOT upgrade to 5.9.2 and stay with 5.9.1 code.
No further devices are available though.
For more details, please contact your distributor or sales representative.
Regards,
Ondrej