18:05 would not be an "illegal" connection, i.e. a connection time shorter than MCT for ARN - I just checked this on SK web-page.
May I ask you, why did you not book the 18:05 in the first place rather than 19:00?
Anyway, it appears that you ticket is flexible so just call SK and ask to be rebooked. Should work - there is availability in J class - I just checked.