It'll change it pretty significantly. The ale yeast and warmer fermentation will cause some esters to form, and it won't be the clean, crisp profile of a lager.
Still, it can be very good! If you choose a "clean" ale yeast and ferment it at the coolest end of the optimal range, it'll be a good lager mimic. I'd use something like wlp001, wyeast 1056, (liquids) or s-05 for dry. Use a big starter for the liquid yeast strains, so you don't start out with stressed yeast, and ferment at 60-65 degrees at the most if you can. That should give you a lager-ish ale.
What is the recipe?
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