Bloomberg reports Wednesday that the number of job openings in the US rose in July by 178,000 to 3.04 million.

This follows last week's report that companies added 67,000 jobs in August, above consensus forecasts, following a 107,000 increase in July.

As the job market picks up, companies won't just be hiring the currently unemployed. They also will be employing those who already have jobs.

And according to a new survey, this latter pool could rapidly grow as the job market improves.

GfK Custom Research North America, a research firm, said about 20 percent of currently employed Americans are willing to change jobs if given the opportunity, according to a survey of 533 employed adults.

"This number is important to watch as the economy continues to improve," said Thomas Hartley, Vice President of GfK Customer Loyalty and Employee Engagement. "Employees are more likely to stay with their company during lean times, and so companies have taken them for granted."

They sure have. Many companies have piled on additional responsibilities, often those held by laid off workers. Many employees have felt intimidated from taking time off or leaving early for family events.

When the job market improves, these are the employees who are first to leave. And those companies that have most aggressively exploited the tight job market will become the most vulnerable to turnover.

"When the economy strengthens, experience tells us that employees will begin weighing their options and considering other jobs," Hartley adds.

Companies know it costs less money to retain employees than recruit new ones. Turnover means additional costs related to search and then training. It also takes new employees some time to get up to speed and operate at the level of previous employees, especially the good ones.

So, now is a good time for companies to honestly assess how happy their rank and file is. Are they over-loading their workers? Are they intimidating them from taking time off?

If so, now could be a good time to lighten the load, encourage employees to take a day off. Throw a "recovery" party.

It could be the best investment they make over the next six to 12 months.