There are no legal issues. If you are under-withheld (by claiming too many exemptions and adjustments) you can be subject to a penalty if you owe too much when you file. Having too much withheld is just an interest-free loan to the IRS.
Yes, changing one W-4 to single will increase the withholding. If you want to know exactly how much, the withholding formulas are in publication 15-T, although it will help if you are an "extra-mathy" person.
https://www.irs.gov/pub/irs-pdf/p15t.pdf
In general, I would suggest that whenever your wages or jobs change, you may want to use the IRS withholding calculator.
https://www.irs.gov/individuals/tax-withholding-estimator
It does require at least one paycheck at the new job or new wage, but it will tell you what your tax position will be at the end of the year if you make no changes, and will suggest changes you can make if you want a larger or smaller refund.
