If the tenant is refusing the new price, may I organise visits with other potential tenants during this month?
In order to terminate a month-to-month lease agreement (and also if you want to substitute in a new month-to-month lease agreement with different terms) you must send notice via certified mail and the other party must receive it with at least 15 days' notice.
So, if the tenants pay rent on the first of each month, you would have to send them a letter terminating the current lease by certified mail that they would have to receive by February 14, 2013 in order to have it terminate at the end of the month.
In the same letter, you can let them know (if you wish) that you are offering a new lease agreement with a higher rent amount and that if they want to accept, they need to let you know by February 20th in writing or you will being advertising and showing the property to new renters.
You can give them proper notice according to your written lease agreement or statutory law that you are coming on x date at x time to show the property.
In a month-to-month, a lease is renewed each month. To terminate a lease, absent any writing to the contrary, you need to give 15 days notice prior to the end of the lease term (end of the month). The same would hold true for price increase, although if they are good tenants, then I would advise giving ample notice of a price increase.