first off, I consider myself pretty new to vaping still but i can tell you that the vivi nova 3.5ml clear tank V2.5 is very easy to use (510)
UK link
and you can just pour the remaining juice back in the bottle without losing even a drop of juice. put in the new juice and vape away the first vapes may be the old juice because its still in the wicks but wont take long for the new juice to kick in (if you fill the tank completely) you can get over 4ml in it

don't know if its good to do so tho
cleaning guide:
The only thing I can say is to clean everything occasionally (I did my first clean after 4 days use). Disassemble the works. Top/bottom cap, drip tip, remove the head and the top rubber cap for it. Rinse everything under warm tap water; or fill a large bowl with warm water if you're worried you'll drop a part down the drain. Dry everything you can get to easily with paper towel, leave it disassembled on a paper towel overnight/24 hours. Put it all back together, fill, and you'll be good to go. The first few pulls may be sub-par if there's still a bit of moisture on the wicks. Don't worry, it cooks off and you'll be back to vaping properly in no time. Personally, I don't even let it wait overnight, though I trim my wicks down so they don't retain as much water. I put it together right away, fill, and just deal with a few "meh" vapes. Once the liquid wicks to the coil it resumes full functionality.
A full disassemble/clean only takes around 5 minutes.
(cleaning guide quoted from
recidivus )