I don't see changing the times as practicing medicine without a license. Changing the frequency would be an order change, but what times? Unless the doc writes for a specific time, and there are few who do, we time the meds for when it is reasonable and appropriate to give them.
Example: admission arrives at 0100 with orders for some BID meds. Now the patient is awake, no problem. I will consider what those meds are that possibly he missed the evening dose, before giving them, because we're about 6-7 hours away from the morning dose. There are things that although they might be BID, I don't want to give that closely together. And I don't control admission times so I am not responsible for the fact that he was ordered meds at 2100 in the admit orders and arrived on my floor well after midnight. This is just an example mind you.
The admits always do give me problems. If they arrive dinnertime or later, whether or not they got their meds that day, I am not going to attempt to give a whole day's meds just in the evening/overnight. With my patients it just sets them up for low heart rates, bottomed out blood pressures, etc. either in the middle of the night or the next AM after the normal dose is given around 0800/0900. So, no thanks.
And what about those "Q 6 hour" and "Q 8 hour" dosed meds? eh? So if your patient's lopressor is due and you hold it for a heart rate of 44, would you give it the next time he's up and about with a decent heart rate, or wait 6-8 hours until it's due again and then check HR/BP to see if you can give it? The docs I work with didn't sit down and write exactly what time we had to give the meds, and it exasperates the cardiologists if a nurse holds the meds all the time because they only check the VS when it says the meds are due on a sheet of paper... I really hope you don't call that bit of nursing judgement and time management, practicing medicine without a license. Good grief. If that's what it is, omygosh, better call the state board 'cause we're all in trouble!
Edit: Ok I found the flaw in your statement. "If the institution has a prescribed time" is the flaw. The institution does not "prescribe" medications, the doctor does. The pharmacy prints times that they think are reasonable for administration based on mealtimes and when people are likely to be awake. That has absolutely ZIP to do with the act of prescribing a drug. Adjusting what time to give a med is well within the boundaries of nursing practice.