No it won't. Your offer that you get on the 18th is final (but you must make sure that you accept it by the date specified by the uni, otherwise you will lose it).
The change of preference period that opens up after round 1 offers is for round 2. If you want to be considered for other courses, you must ensure that your order reflects what you want to be considered for in round 2. In this case, it would be perfectly okay to move your offered preference to the bottom of the list (because this will be marked as Offered in round 1 anyway).
If you don't move your offered preference, only courses above it in the preference list will be considered for round 2.
Remember: Always accept your first round offer - you are able to withdraw from it later on if you get offered something else in future rounds. By not accepting your first round offer, you run the risk of not getting ANY FURTHER OFFERS. Most offers are made in round 1 - and as the rounds continue, the likelihood of you getting any offers decreases.