Kenneth Branagh's goal in adapting Shakespeare's work for the screen over the years has been making the Bard's stories more approachable for a modern audience. The 1993 Much Ado About Nothing is one of the most successful Shakespeare movies of all time, no doubt due in part to the way Branagh approaches the setting.
The original play and the adaptation are both set in Italy, which lends a romantic air to the proceedings. The original's temporal setting was contemporary for its late sixteenth century audience, but because it was set in Italy, it would have been foreign enough to act as escapism for an English audience.
Branagh sets the film in Tuscany during the summertime, creating an appealing backdrop for the romantic intrigue and comic banter of the story. It too transports its audiences in an escapist fashion. However, he makes the temporal setting rather vague, even dreamlike. The costumes of the characters seem reminiscent of eighteenth and nineteenth century fashions, and sometimes even Renaissance era styles, but there is nothing specific to tether them to any particular period. This is Branagh's way of setting the story out of time, suggesting it belongs to all time, anywhere.
The message of the play is much the same. Branagh does not change too much, as he does in his adaptation of Henry V, where Henry's darker side is allowed more free reign, playing down the more nationalistic elements. The themes of Much Ado About Nothing have little to no problematic elements for modern audiences, relying on more universal themes of misunderstandings, infidelity, identity, and love. Branagh does cut the play down a little, but this is more for cinematic convention than altering much thematic content.
So, I would say that while the alteration of the setting's time period from specific to vague is meant to help a modern audience see the story as timeless and relateable to their own lives in a storybook sort of way, it does not change the message of Shakespeare's play very much, if at all.