If you follow the 5E rules and simply pull monsters (or use the monster creation rules in the DMG to recreate monsters from the 3.5E source books), magic items, and other things from the 5E books, and pull the lore (fluff) from the 3.5E source books you'll end up fine.
If you use the monster, magic items, and other stats from the 3.5E source books, it will be a disaster.
Fluff; (Noun) 1. Descriptions, lore, or non-rules information.
Crunch; (Noun) 1. Rules, statistics, or numbers.
A more detailed look:
- You will have to recreate any encounters by using the DMG encounter building rules.
- You will probably have to redistribute treasure and magical items using the 5e rules in the DMG.
- You will have to use the 5e monster statistics or recreate monsters using the DMG monster creation rules.
- You will have to adjust skill checks to use 5e skills and DCs.
Example - There is a room that has a pit to Athletics[jump] (DC 25) and 12 Orcs with short bows on the other side, one of which has a magical +3 compound short bow of strength 14 and all of this is for a party of five level 3 characters. You will have to change the jump check to Strength (Athletics) DC 15, the 12 orcs will turn into 3 orcs (or you can create orc youths that are CR 1/8 and have 5-6). The magical short bow would not exist or would be reduced to a short bow +1.
The fluff of the encounter is that there is a pit with orcs on the other side with bows one of which is magical. The crunch is DC 25 jump check, 12 orcs, and compound short bow +3.
TLDR; Earlier edition fluff = good. Earlier edition crunch = bad.