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.
Best Answer
Give 'em enough rope, but also provide clues and hints
You need to determine, by talking to the veteran players, whether or not they are aware that they can flee if a fight is too much for them. Do this before the next session begins.
This danger in short cutting to Cragmaw (before you think they can handle it) needs to be shown in the shape of warnings, forebodings, rumors, and some physical evidence found (dead bodies, discarded equipment, etc) as they approach "the end" prematurely.
If they don't heed the clues, let them face a lethal encounter and ...
... roll up new characters after the TPK or
... awaken as prisoners of (pick an NPC) with their next challenge being ... how do we get out of here? or
... surprise you by succeeding against extremely long odds
A reason I raise point 2 is that with new players, and given how swingy low level combat is, the option to proceed-on after failing is a way to encourage new players to learn as they play. So too is the party wipe: it really depends on your players.
If you have not yet discussed with them, out of character, whether or not a TPK may happen if they don't use their wits (in a general sense) then discussing this with the whole group before Session 2 begins is a good idea. You might call that a bit of out-of-character foreboding, but with new players being in the mix that is not necessarily a bad thing.
Option 2: transfer the knowledge to a different NPC
You can decide to not raise that point during the interrogation and have a different NPC, later on, provide the same info that the goblin has now. That information does not have to come to the players from this goblin. It can come from any NPC; you are the DM, pacing is within your area of control.
I just noticed that @Neewbie12345 covers this option in more detail so I won't elaborate further.