While most of us want to upgrade to become better in our chosen fields, sometimes veteran/uber players come in a situation where they temporarily have less time to invest in EU. Whether it be a job opportunity, a girlfriend or just RL business absorbing you. You feel the need to downgrade a bit, an here is a guide to it.
Just remember that this guide is time dependant, the situation may be completely different in a years time.

First thing you must decide is how much and in which way you will be playing. I highly recommend still participating in special events like MM, since this is the money shot for you veteran/uber player.

Secondly, if you own CLD or LA's; it is a given you keep them. It takes relatively no time to re-fertilize your lands and CLDs are no work at all. Money will continue to flow in.

Thirdly, gear.
Now it is not because you max all those nice oldskool weapons, that they are the best thing to hold onto. There is only one direction the price will go, and that is down. Is it really worth to keep them if you won't use them?
And if you look at the high end L weapons, the markup is so low, and their DPS is higher than most oldskool weapons.

Do you really need 5 sets+ of armor? With current plates and many L armors at decent prices, you might only keep one or two sets and sell the rest. If you casually log in, you can look for cheap deals on uber L armor for times of need (events) while still having one or two decent armors to have fun.

Do you keep your Imp/mod Fap? Well actually, this is the only tool (next to mining amps) that will hold its value. So you might want to hold onto this one. Ofcourse if you really need the money IRL, you can sell it. But from everything you might sell, you will regret selling your uber FAP. there is just not any L equivalent ingame.

PK gear is a waste, unless it's your hobby. PK'ing is actually waiting till someone dumb enough wanders into toxic zone with too much ore/hunting loot and shoot them. It is quite boring actually, and what is the return really?
if you shoot one 50pedder in an evening you are lucky. so Tango, eon and the likes are better off to someone bloodthirsty and with more time.

privateer, MS and the likes; well, tell me, how much will you swap planets and you have time. You can wait till some MS passes by and go for 10ped to another planet, without fear of being looted. no warp drive decay and the likes.

four, skills. If you are planning to use lvl80+ weapons; is there really a need to more than max oldskool gear. Face it, all skills that don't contribute to HP are irrelevant if they only contribute to lvl100+ in your profession. It might be a status symbol, but you're becoming a more casual player, and skills aren't going up in value anytime soon.

five, what to keep? Well, if you aim to play casually and mostly events like MM; the things to keep are simple. Sell your Dante, keep your Evil amp. look for cheap high level L guns with high tiers and have fun tiering them up on sundays while waiting for MM or some other event to come by.
You might also be on the look for L uber armor. Every now and then some parts sell cheap. Actually this counts for most stuff; sometimes you'll see just the thing you will need in a few months for a great price.
Also stock on L amps (hunting) when you find cheap ones. You might also want to stock up on dmg enhancers between events like migration and such, when the price drops. Don't wait till the event, prices do skyrocket and miners and crafters reap the profits.

Six keep in touch. Even if you play only once a week; do ask what has changed, what's new and have fun. Believe me if you only casually play, the game is more fun. But don't forget to keep up with things.

Seven know that every piece of gear you sell will make someone happy. It will create the ubers of tomorrow. It's okay to get a little sentimental over it.

Eight and what if you want to upgrade again?
Well, prices for oldskool will be down, you still have your uber fap and you still know what goes around. You might end up buying stuff back at only a fraction of the selling price. Or buying much better stuff that didn't exist before, at the price you sold your old and now obsolete gear.
And most of all, you will have turned into a legend in your absence. And who knows, if you have invested that money IRL, you're buying that stuff with the profits of your ever-growing investments.