What can I do about having to stand up all day during work if I have lymphedema and vein insufficiency?

Treat the veins. I would recommend that you wear graduated compression stockings and have the veins treated by evla or vnus and foam sclerotherapy. I generally recommend that you wear thigh high stockings with a compression of a minimum 20 to 30 mmhg (class i) but prefer the 30 to 40 mmhg (class ii). Treatment of the underlying venous insufficiency will likely improve the swelling.
Underlying cause?? Check for saphenous insufficiency first! it is done by a special person trained to look at veins by ultrasound.
Take care of both. If you have both lymphedema and venous insufficiency, treating your veins will probably help your swelling some but it isn't likely to make it go way entirely. Wearing graduated compression stockings will help but seeing a lymphedema therapist is also something you should look into.
Compression. If you have lymphedema and venous insufficiency (leaky valves in your veins) the best thing you can do is get a pair of below knee compression socks. There are various grades / strengths. The higher grade stockings (> 30mm hg) are better in theory but are difficult to put on and not comfortable. The lower pressure stockings (15-30mm hg) can be worn like normal socks and are more comfortable.