If you come here and waltz through immigration with your COE/work visa, then you have to set yourself up with housing and look for the job you really want. Housing rentals require a guarantor, typically your employer, so you won't have that, and you'll have to live in something shorter term in the meantime.
But as Mike wrote, that next employer should see that you have no work experience in Japan under that work visa, and they should raise an eyebrow at what it means. Eventually, someone will hire you, but I side with Mike in thinking it won't be any better than the place you just decided to abandon, precisely for the reason I just stated -- you look unreliable. Plus, you make foreign teachers like the rest of us look bad. Thanks in advance. (sarcasm mode on, there)
If you don't want this first employer, quit now, cancel the COE, and start anew. If you got the COE with little problems the first time, it will be the same the second time.
When do you want to come?
What's the rush? (2nd time I've asked this)