#1
hey, would it be better to upgrade my epiphone LP-100 with some very good pickups and whatever else or get an LTD EC-400 AT
#2
If you need a second guitar then buy another, but if you can manage with just one one then an upgrade would be cheaper and make more sense
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#3
Would it be better? I am sure that a new LTD EC guitar would be better than an LP100 with new pups.

Guitar fetish has good cheap pups if your interested.
#4
Personally, the LP-100 is just a beginner axe.
Overall, even if you did upgrade the pickups, it's still a beginner axe. Aim for something a little higher.
My newbie stuff:

Ashton Acoustic (forgot model :P)
Squier Stratocaster
Fender Frontman 15R

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Saving up for...

Vox Valvetronix ADVT50
Boss D-2 Pedal
Les Paul Studio (Wine Red)