The specific wording of
You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it
Should overrule the general wording of
You can then dismiss the weapon, shunting it to an extra-dimensional space, and it appears whenever you create your pact weapon thereafter.
"it" being either a reference to the magic weapon or the Pact Weapon, which can take on any form of weapon you choose (especially with Improved Pact Weapon). So by the rules of 5e, you could do it. However...
Sage Advice, the rules editing body of WotC, made ruling on this subject and says that you can't do this. Here is that ruling:
The warlock’s Pact of the Blade feature (PH, 107–8) lets you create a melee weapon out of nothing. Whenever you do so, you determine the weapon’s form, choosing from the melee weapon options in the Weapons table in the Player’s Handbook (p. 149). For example, you can create a greataxe, and then use the feature again to create a javelin, which causes the greataxe to disappear.
You can also use Pact of the Blade to bond with a magic weapon, turning it into your pact weapon. This magic weapon doesn’t have to be a melee weapon, so you could use the feature on a +1 longbow, for instance. Once the bond is formed, the magic weapon appears whenever you call your pact weapon to you, and the intent is that you can’t change the magic weapon’s form when it appears. For example, if you bond with a flame tongue (longsword) and send the weapon to the feature’s extra-dimensional space, the weapon comes back as a longsword when you summon it. You don’t get to turn it into a club. Similarly, if you bond with a dagger of venom, you can’t summon it as a maul; it’s always a dagger.
As both a player and a DM with over 4 decades of playing under his belt, both D&D and many other systems, I would disregard this! This ruling does nothing except stop players from adding flavor to the role-playing aspect of the game and making the game more fun. In all of the games I know of, it has next to zero effect on game mechanics, but allow players to have some fun with their Blade Pact Warlocks. Some have noted that it changes the type of damage, but if you look at the listings in the MM, all three are grouped together 99% of the time, as all 3 are resisted unless they are magical weapons. The 1/2 dozen that actually specify one type are very rare, and since it takes a full round to change the weapon into another form, the penalty is that the Bladelock will loose an additional turn before acting, which more than balances out the 6 or so times it is relevant.
There is nothing wrong with a Maul of Fish Command, a Whip of Venom or a Holy Avenger Morning Star!
The original argument was to keep Warlocks from wielding "great" weapons as their pact weapons when they didn't have the proficiency to do so, but with the Improved Pact Weapon and the Hexblade Patron, they can form "great" weapons as pact weapons, but also bows as pact weapons too. So the original argument has been wiped out by an Invocation and/or a Patron. The only thing this restriction does is take away role-playing flavor. I hope they will see the error of their ways and reverse the ruling in a future update of the Compendium.
Exception: If you have a DM who revels in giving magic items to the party they can't use, or deliberately giving weapons that don't fit the player's vision of their character, then, yes, this does throw a monkey-wrench in the works for those types of DMs! In those cases, I wish they would reverse the ruling, specifically because doing so is far better for role-playing aspect, the very basis of the game, but it would also have a positive impact, improving the games where the players are at a table with that DMing style!