The difference between 3.5e and 5e mechanically - are pretty great.
Particularly as 5e introduces the "advantage / disadvantage" mechanic,
which doesn't really have a close equivalent in 3.5e (yes, the Luck
domain power can let you reroll a d20, and there's some other d20
trickery around as well, but it isn't really the advantage /
disadvantage mechanic).
On page 3, LMoP says that DC10 is Easy, DC15
is Moderate, and DC20 is Hard. That's fair, and should work fine in
both 5e and 3.5e. However, as you're going towards 5th level with the
player characters (LMoP is meant to go from 1st to 5th, roughly), you
may see that 3.5e increases in a different way as compared to 5th
edition.
A 5th level 3.5e character can be expected to hit a bonus of
about 10 - 15 pretty easily in a class skill, (+2/+3 from attribute, 8
ranks, perhaps a Synergy +2 and a Misc +2) and will therefore start
hitting higher DCs consistently as compared to a 5th level 5e
character. At 1st - 3rd level, I'd say the difference between 3.5e and
5e when it comes to DCs, is largely something you can ignore.
Goblins
are goblins. Replacing them should be fine. Treasure is treasure - but
I should also caution that I find 3.5e is more item-driven than 5e, so
you should take a stock of things when LMoP is done, and perhaps up
the treasure slightly at 4th and 5th level (slightly more potions,
perhaps a scroll or two more, maybe slightly more powerful arms and
armor).
Not that monsters - in particular undead and plant-based
monsters - have different immunities in 5e as compared to 3.5e. The
humans and orcs can be used much as they're written up in the module;
you don't really need to do much there at all. Same goes for the ogre,
and most other humanoids in the list.
I suspect the immunities and
resistances of the dragon are quite different as well - but it's not
an immediate issue, and you can look at it when you get there.
Attack
bonuses start diverging once you level up a couple of times as well,
but for NPCs it doesn't really matter much between 1st and 5th level.
For NPCs of higher HD, you might want to recalculate attack bonuses
though, based on their BAB and so on, in order to keep things
challenging.
XP is a bit of a sore point. If the party starts lagging
behind the suggested power curve, make sure that you're giving out
enough XP. Also, consider giving out bonus XP (a hundred here, a
couple hundred there) for particularly good and/or entertaining
roleplaying. An RPG session shouldn't be death by spreadsheet - it
should be fun, whichever system you're using.
So in short - yes, you
can keep the DCs as suggested, you can keep the monsters pretty much
as suggested, but you need to look at resistances and immunities, and
towards the higher bracket you need to look at attack bonuses. You
should also keep an eye on the treasure given (and possibly increase
it), and if the party starts lagging behind you should consider upping
the XP given.
