401k enrollment date versus contribution change date

I feel like I should know the answer to this but it is not coming to me.  If a plan has semi-annual entry dates, Jan 1 and July 1, and a participant does not enroll on January 1, can they still sign up for the plan later in January because the plan allows contribution changes at any time?  

Do we take the position that the individual is changing from 0% to 5% (or whatever) and that is allowed any time?

Or do "contribution changes" refer to something other than changing from 0%?

If the Plan allows changes at any time then yes, they can go from 0% to greater than 0% at any time.

Thank you!