> 1. and build upon that, but **build** they have. -- [original]
> 2. and build upon that, but **built** they have.

In general, both versions are acceptable in today's standard English.

But in this specific example, there would often be a preference for ***version #1***, which is the original version that had used "build":

* 1. *and **build** upon that, but **build** they have.* -- [original]

Version #1 would often be preferred because the second "build" would then match the first "build" which was used in the previous clause, and that would give a rhetoric effect which would often be desirable by the speaker.

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LONG VERSION

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Your example uses **complement preposing** in the second clause, where the preposed element is a verb phrase (VP). Usually the second clause will involve an auxiliary verb when the preposed element is a VP. 

Here are some typical examples. The 2002 *CGEL*, page 1376:

> * [11.i ] _I've promised to help them [ and help them I will ]._

> * [11.ii ] _It's odd that Diane should have said that, if [ say it she did ]._

The preposed VP in [11.i ] is "help them", and in [11.ii ] it is "say it". Notice that the nucleus of the second clause in both examples end with an auxiliary: "will" for [11.i ], and "did" for [11.ii ].

Here are their corresponding versions that don't have the preposing:

* A.i. _I've promised to help them and I will help them._
* A.ii. _It's odd that Diane should have said that, if [ she said it ] / [ she did say it ]._


But when the auxiliary verb is the **perfect** "have" and the preposed element is its complement, then both the past-participle form and the plain form of the verb are acceptable.

The 2002 *CGEL* page 1381:

> **Inflection with perfect *have***

> A special issue arises when the preposed element is a complement of perfect *have*. Compare:

> [25]

> * i. _He said he wouldn't tell them, [ but tell/told them he has ]._

> * ii. _He denies he has told them, [ but tell/told them he has ]._

> Although *have* normally takes a past participle, it is the plain form of the verb that is preferred in [i ]. The past participle is preferred in [ii ], where it has been used in the preceding clause, but even here the plain form *tell* is acceptable.


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NOTE: The 2002 CGEL is the 2002 reference grammar by Huddleston and Pullum (et al.), *The Cambridge Grammar of the English Language.*