You haven’t said “why” you want more aperture; is it because you want more resolving capability for the moon and planets or because you want more light-gathering capability for deep sky objects or both?
For the moon and planets:
If you live in an area with good to excellent seeing on many nights of the year the move will be productive. If you live in an area where the seeing is average or below on most nights of the year then the move might not be as productive as you would like.
For deep sky:
More aperture is always better but the jump from 155 to 180mm is not that great. Will there be an improvement? Yes. But that improvement might not be as dramatic as you would like.
If you are unsure about your seeing then buy a cheap 10” Dobsonian and use it for a year on the moon and planets. If there are enough nights where the 10” shows an improvement to your satisfaction, then the 180 would do even better. If not, then seeing in your area might not justify the 180 to your level of satisfaction.
For deep sky:
Again that 10” will give you an idea of the level of improvement. If you like what you see, then the 180 would be worth it. If the 10” does not deliver enough of an improvement to satisfy then neither will the 180.
After the evaluation:
if you are convinced that the 180 will deliver enough to satisfy then you can easily sell the 155 and the 10”. If after the evaluation you are convinced that the move will “not” deliver enough, then you can sell the 10” and you have saved a ton of money by not spending thousands on the 180 while also having departed with the wonderful AP 155.
Something more out of the box:
If you are happy with the 155 for the moon and planets but want more deep sky penetration, I would strongly consider an image intensifier. An intensifier is as easy to use as an eyepiece and will deliver deep sky views that few if any telescopes used with regular glass eyepieces will match. An intensifier and an AP 155 on an alt/az mount will be a true, deep sky, killing machine! Objects like the Horsehead Nebula will be child’s play.
If you have already evaluated your seeing and you are sure the 180 would deliver more to your satisfaction and you are sure you can handle it then great – go for the 180.
For more information about image intensifiers, go to the Night Vision forum. With your 155 or possible 180 an image intensifier is highly recommended. You get a “huge” light-gathering boost with "no portability penalty".
Keep in mind that telescopes "get heavier" as we age.