New claim forms for seeking possession of a house or flat let to a tenant under an assured shorthold tenancy are to be used as from 1 December 2017. Because of changes in Welsh housing law, separate forms have been devised for proceedings in England and proceedings in Wales. The defence form has been revised too and with different versions for England and Wales. The claim form for England is called the N5B England and the claim form for Wales is called the N5B Wales. Cool.
The new forms can be accessed on http://formfinder.hmctsformfinder.justice.gov.uk
If you use the old form on or after 1 December 2017 it is likely to be bounced back to you. If it isn't, you could well be refused a possession order when the judge takes a look at it and have to amend your paperwork. No problem will arise when completing the claim form online because you will be completing the correct form.
There are numerous traps for a landlord seeking a possession order and as many opportunities for tenants to deny their landlord possession because they got it wrong. For these and the landlord pain and tenant joy of a penalty over failure to protect a deposit, see my book Breaking Law.