Dungeon of the Mad Mage

This is a new answer: since this question was originally asked and answered in 2016, new information has become available. An upcoming adventure releasing November 13th, 2018 titled Waterdeep: Dungeon of the Mad Mage will run from Level 5 to Level 20.

According to the product description on Wizards of the Coast's official D&D website:

This adventure picks up where Waterdeep: Dragon Heist leaves off, taking characters of 5th level or higher all the way to 20th level should they explore the entirety of Halaster’s home.

That will make this the first official WotC-published D&D 5E module that takes players to Level 20.

Wizards of the Coast posted a short 4 page article about conversion from previous editions of D&D to 5th edition. It covers monsters as well. You may even convert those from 1st or 2nd edition. I find it quite helpful.

Conversions to 5th Edition D&D:

Quick Conversion

In third edition, you can use monster statistics included in an adventure as a guide. Monster distribution in this edition is fairly close to the distribution in fifth edition. As in earlier editions, such creatures often deal lower damage and have fewer hit points than their fifth edition counterparts. Most statistics in third edition include the creature’s ability scores. Use the following parameters:

• Armor Class can be an average of touch AC and actual AC, or 20 percent lower than in third edition. The upper limit is 22.
• Attack roll modifiers are the appropriate ability score modifier + 3.
• Saving throw DCs are 10 + the appropriate ability score modifier.
• If a creature has to make a check or saving throw, use its ability score modifiers. Grant it a +3 bonus if it should be good at the roll.

Careful conversion

In older adventures, you’ll sometimes run across creatures that lack a published equivalent in fifth edition. In this case, the “Creating a Monster” section in chapter 9 of the Dungeon Master’s Guide is your best tool. The “For the DM: Race Conversions” section in this document can also be helpful.

If you choose to create the monster whole cloth, start by replicating the original monster as closely as possible using elements from fifth edition. Use the HD as the creature’s HD in fifth edition (the level of a fourth edition monster can determine the number of HD it has). Ability scores should be in the fifth edition range, from 3 to 30; third and fourth edition ability scores often need to be reduced. For special traits and attacks, use existing creatures as guidelines.

Once you’ve converted a monster to fifth edition or created a new one, you can use the Monster Statistics by Challenge Rating table in the Dungeon Master’s Guide to determine the creature’s challenge rating. Then, you can adjust HD and other statistics to get the challenge rating you desire.